From Battleground Europe Wiki

Infantry unit restrictions are outside of the RDP tiers and use Rank instead. They are still subject to the rules of Supply. Thus, if a unit is available, you may not be able to access it due to your rank, but if the unit is unavailable, you cannot access it regardless of your rank. Previous to 1.30 the structure only offered the Rifle in rank 1, the SMG, Sapper, Grenadier, LMG, ATR in rank 3, Paratrooper and Sniper in rank 5, and Binoculars with Mission posting in rank 6. The new structure is below.

Rank	Unit
1	Rifleman, Binoculars
2	SMG
3	Sapper
4	LMG, ATR
5	Mortarman, Grenadier
6	Mission Creation
7	Sniper

Paratrooper Infantry

same for all tiers

Rank	Unit/Ability
1	Para Rifle
2	SMG
3
4	LMG/ATR
5	Mortarman, Transport aircraft
6	Mission Creation
7	Sniper