[Mobhack]

Unit classes exist for OOBs - if you only want a certain type in the mech companies, the leg companies, commando companies etc. Some unit classes have extra attributes - see the Game Guide (e.g. mountaineers are less affected by rough/impassable terrain for movement). Some classes are better as paratroopers (the para and commando classes) - ordinary guys thrown out the back of a plane will have more landing casualties.

There is basically no difference between a "heavy" section and a "light" one bar the name and the unit class. the heavy is typically used for e.g. a British platoon with the HQ and weapons section, the other 3 being of another class. Thus the HQ section has the PIAT and 2in mortar, the rifle sections have the regular platoon weapons.