Platoon

Platoons are the third smallest infantry units in the UNSC Army, Marine Corps and Spartan-IV Regiment. Platoons are usaully led by commissioned officers with a non-commissioned officer as an assistant commander. A platoon is roughly equivalent to a Covenant Army group. A squad is immediately below a platoon, while a company is immediately above.

More information at: halopedia and wikipedia.

Background
Platoons are units made up of either twenty, forty-one or forty-two men, depending on the branch. Platoons are usually made up of three to four squads or sections.

UNSC Army
An army platoon typically contains three nine-man rifles squads (wielding MA37 ICWS rifles, M392 DMR rifles and M247 GPMG machine guns), one nine-man weapons squad (wielding M41 SSR launchers and M247H machine guns) as well as a commanding officer, a platoon sergeant, two radio operators, a forward observer and a medic. The commanding officer is typically a second lieutenant and the platoon sergeant is typically a sergeant first class. The radio operators, the forward observer and the medic are all usually sergeants and corporals. All of the platoon staff usually have MA37 ICWS rifles.

UNSC Marine Corps
A marine corps platoon typically contains three thirteen-man squads (wielding MA5B ICWS rifles, BR55 rifles and M247 GPMG machine guns) as well as a second lieutenant as platoon commander and a staff sergeant as platoon sergeant. In weapons platoons, one squad will wield M41 SSR launchers, another will wield M247H machine guns and the last will wield M6 G/GNR Spartan Lasers.

Spartan-IV Regiment
A Spartan-IV Regiment platoon typically contains three six-man sections. Along with these sections, there is the platoon commander and platoon sergeant. As SIV units are mixes of personnel from different branches, the ranks of members would end up different from others (but staying as E-5 for platoon sergeant and O-1/O-2 for platoon commander). In any case, the platoon commander will usually be a second lieutenant, a first lieutenant (Air Force/Army/Marine Corps), an ensign or a lieutenant (junior grade) (Navy), while the platoon sergeant would be a staff sergeant (Air Force), a sergeant (Army), a sergeant (Marine Corps) or a petty officer second class (Navy).