If you just do a little survey of the various Armours/Dress/Suits/Units available via ArmorMaker you can see that there are:
Cold
Combat Environment
Drop
Hazmat
Hostile Environment
Hot
Vacc
All of the above say to me "hazardous environments" and might form the basic equipment of Protected Forces. The use of Units might be useful to Protected Forces on high gravity worlds. I'd also see them as using ATVs (and other vehicles) with the double sealed option from VehicleMaker that means the vehicle is equipped with an airlock.
Protected Forces Characters might focus on skills such as High-G, Hostile Environment, Vacc suit and Zero-G alongside the Soldier Skills.
Purely IMTU i put Drop Troops in the "Protected Forces" category for my Imperial Army. Jumping out of a ship or using a drop capsule for atmospheric re-entry counts as special training and hazardous environment. I use the Rapid Interface Battalion model from GT: Ground Forces which are essentially Light Infantry but I use the Travellers Digest Drop Regiment model from the Imperial Guard article so its a mixed unit of three light infantry battalions, a grav tank battalion and an artillery battalion (I'm a bit inspired by the Russian airborne forces which are also a separate branch). So my Drop Troops Regiments span the Infantry, Cavalry and Artillery Branches plus have specialist training and environmental abilities. Protected Forces also include special troops raised for specific environmental challenges such as Rad Worlds, Space Habitats, unusual atmospheres etc.
IMTU Protected forces in the Imperial Marines are that bit more specialized because all Marines can fit the "Assault from Space" model that Army Drop Troops do, but Marines put more emphasis on the ability to function in zero-g when used as ships troops and operations in vacuum when used in boarding actions. So Marine Protected Forces are those who routinely function inside and outside ships and space habitats.