[Suhiir]

In any formation the "zero" unit is the command unit.
The actual rank of the "zero" unit leader is semi-irrelevant.
Other then in that the highest ranking unit leader in a formation will command the formation.

For example: A platoon formation has 3 units, all squads, B0, B1, B2. Normally B0 is commanded by a lieutenant and squads B1 and B2 by sergeants.
If however B0 was commanded by a sergeant and B2 by a lieutenant then due to rank B2 would be the formation commander, even tho it is not the "zero" unit in the formation. This would effect the ability of the "zero" unit to rally the units (note the "zero" unit also pretty much automatically has a radio and can call in and adjust fire support, if it's not the command unit there may be "issues").
If B0 were commanded by a sergeant and B1 and B2 by corporals the platoon would function as normal for rally purposes.

If a formation has sub-formations (i.e. a company formation (B0) has platoon sub-formations (C0-C2, D0-D2, E0-E2)) then the formation command unit (B0) will be the first to attempt to rally each sub-formation unit (if it is in command range) followed by the sub-formation (C0) leader, then finally the unit (C1) leader. Sub-formation leaders D0 and E0 cannot rally units that are not in their sub-formation.
Again, if B0 were commanded by a sergeant, C0/D0/E0 by corporals, and the units (C1, C2, D1, D2, E1, E2) by privates the formation and sub-formation rally chain would work as normal.

I hope this is more helpful then confusing.