Platoon
From Halopedia, the Halo wiki
Introduction
A Platoon is a military unit used by the UNSC Marine Corps to organize its combat forces. Normally, a Platoon is composed of several squads or Fire teams. A platoon can atleast contain between 30 and 50 members.
Composition
Platoons are derived from Company which are from a Battalion. Each Battalion makes up a Regimant to form a Division. Depending on their purpose, a Platoon can have a varying number and specialty of its personnel. Using a US Marine Corps platoon as an example, a Rifle Platoon usually encompasses three squads of riflemen, as well as a Navy medical Corpsman, a Platoon Commander (usually a Second Lieutenant) and a Platoon Sergeant (usually a Sergeant First Class). If the UNSC uses a Weapons Platoon, then the squads are replaced by mortar section, assault section, and medium machine gun section.
ODST platoons seem to have fewer members than the regular Marine units, possibly because they aren't as easily defeated on battle, making it better to deploy them in smaller forces.
