Military rank
A retired rank is usually kept for life, if the officer concerned so wishes. In the Commonwealth of Nations, such an officer will also hold the style of Esquire, if they do not hold a higher title. In the United Kingdom, an officer of the regular service placed on the retired list or on the reserve will be shown in his/her substantive rank. An officer of the rank of lieutenant, captain, flight lieutenant or above leaving the reserve, or having no reserve liability is permitted to use their rank a courtesy title if they have completed 3 years service in the active list.
Veterans rank is different in each country. Members of the United States military maintain their highest rank after discharge or retirement. 10 U.S. Code § 772(e) states: A person not on active duty who served honorably in time of war in the Army, Navy, Air Force, or Marine Corps may bear the title and wear the uniform of the highest grade held by him during that war. After a war, regular serving members of the military holding war substantive or temporary rank often revert to their former, substantive rank and all others often end their service. However, the holder may be granted permission to permanently retain the rank they held when the conflict ended.
Temporary: usually granted for a specific task or mission. The holder holds the rank while occupying that position. Despite the name, temporary rank may be held for a considerable period of time, perhaps even years. In wartime, temporary ranks are often common. In the United Kingdom, the rank of brigadier was long considered a temporary rank; while its holder was addressed as "brigadier," he would retain the substantive rank of colonel or lieutenant-colonel if not selected for promotion to general officer rank. Sub-classes of temporary rank (from highest to lowest) include:
War substantive: a temporarily confirmed rank only held for the duration of that war, though war substantive rank may be treated as substantive when considering the holder's eligibility for subsequent promotions and appointments.
Acting is where the holder assumes the pay and allowances appropriate to the acting rank, but a higher commanding officer may revert the holder to previous rank held. This is normally for a short period of time while the permanent occupant of the office is absent. During wartime, acting ranks are frequently held on an emergency basis, while peacetime holders of acting ranks are often those who must hold their permanent rank for a sufficient period before being confirmed in their new higher rank.
Brevet: an honorary promoted rank, without the full official authority or pay appropriate to the rank.
Local or theater: a form of temporary rank restricted to a specific location instead of a specific duty.
Honorary: Often granted on retirement, or in certain special cases to honour a deserving civilian. Generally, honorary rank is treated as if it were substantive, but usually does not grant a corresponding wage or pension (increase). Certain members of the British royal family that serve in the military have honorary ranks in other services, but retain their original rank. Prince Harry, the Duke of Sussex, is a major in the British Army, but has an honorary rank of lieutenant commander in the Royal Navy, and squadron leader in the Royal Air Force.

Size of command: 

Rank and unit size: To get a sense of the practical meaning of these ranks—and thus to be able to compare them across the different armed services, different nations, and the variations of titles and insignia—an understanding of the relative levels and sizes of each command is helpful. The ranking and command system used by U.S. Marine ground forces or U.S. Army infantry units can serve as a template for this purpose. Different countries will often use their own systems that do not match the presentation here.
Under this system, starting from the bottom and working up, a corporal leads a fireteam consisting of three other individuals. A sergeant leads a squad consisting of three fireteams. As a result, a full squad numbers 13 individuals. Squads usually have numbered designations (e.g., 1st Squad).
Generally, in most armies and marine units, a lieutenant or equivalent rank leads a platoon, which can consist of three or four squads. For example, in U.S. Marine infantry units, rifle platoons usually consist of three rifle squads of 13 men each, with a Navy corpsman, the platoon leader, and a platoon sergeant (i.e., a staff sergeant who serves as second-in-command). An infantry platoon can number from 42 to 55 individuals, depending on the service. Platoons are usually numbered (e.g., 1st Platoon) or named after their primary function (e.g., service platoon).
A captain or equivalent rank commands a company, usually consisting of four platoons (three line platoons and one heavy weapons platoon). His headquarters can include a first sergeant and as many as seven others. As such, a company can comprise from roughly 175 to 225 individuals. Equivalent units also commanded by captains are batteries (for field artillery units) and detachments. In English speaking countries, a company (or troop in the cavalry, and battery in the artillery) is usually designated by a letter (e.g., "A Company"). In non-English speaking countries, they are usually numbered. In most Commonwealth armies a company is commanded by a major, assisted by a captain.
A lieutenant colonel or equivalent rank commands a battalion or a squadron, often consisting of four companies, plus the various members of his headquarters. A battalion is around 500–1,500 men and usually consists of between two and six companies.
A colonel or equivalent commands a regiment or group, often consisting of two or more battalions (for an infantry unit) or five to six air groups (for a wing). Battalions and regiments are usually numbered, either as a separate battalion or as part of a regimental structure (e.g., 1-501st Infantry in the US Army).
The next level has been a brigade, commanded by a brigadier general, and containing two or more regiments or three to eight battalions. In the U.S. Army, a brigade is roughly equal to or a little larger than a regiment, consisting of three to seven battalions. Strength typically ranges from 1,500 to 3,500 personnel. In the U.S. Marines, brigades are only formed for certain missions. In size and nature they are larger and more varied collections of battalions than is common for a regiment, fitting them for their traditional role as the smallest formation able to operate independently on a battlefield without external logistical tactical support. Brigades are usually numbered (e.g., 2nd Brigade).
The level above regiment and brigade is the division, commanded by a major general and consisting of from 10,000 to 20,000 persons. The 1st Marine Division, for example, is made up of four marine regiments (of the type described above), one assault amphibian battalion, one reconnaissance battalion, two light armored reconnaissance battalions, one combat engineer battalion, one tank battalion, and one headquarters battalion—totalling more than 19,000 marines. (Within the headquarters battalion are one headquarters company, one service company, one military police company, one communications company, and one truck company.) An equivalent elsewhere within the same marine expeditionary force (MEF) might be an MEF logistics group (MLG), which is not a regimental-sized unit (as the word "group" implies), but rather a large support unit consisting of several battalions of support personnel. Divisions are normally numbered, but can be named after a function or personage.
The command sizes for any given rank will vary widely. Not all units are as troop intensive as infantry forces need to be. Tank and artillery crews, for example, involve far fewer personnel. Numbers also differ for non-combat units such as quartermasters, cooks, and hospital staff. Beyond this, in any real situation, not all units will be at full strength and there will be various attachments and detachments of assorted specialists woven throughout the system.
In the U.S. Army, the level above division is a corps. It is commanded by a lieutenant general. In many armies, a corps numbers around 60,000, usually divided into three divisions. Corps (and similar organizations) are normally designated with Roman numerals and their nationality when operating in a combined (international) force—e.g., V (US) Corps, VIII (ROK) Corps, II MEF, I Canadian Corps.
During World War II, due to the large scale of combat, multiple corps were combined into field armies commanded in theory by a general (four stars), but often by a lieutenant general (three stars), and comprising as many as 240,000 troops. Armies are numbered by spelled-out numerals or functional titles, using their nationality in "combined" forces (e.g., Eighth United States Army, Third ROK Army, British Army of the Rhine). These were in their turn formed into army groups, these being the largest field organization handled by a single commander in modern warfare. Army groups included between 400,000 and 1,500,000 troops. Army groups received Arabic numeral designations and national designations when combined- e.g. 21st Army Group.