I believe that the ability designation means the following...
Unit:
This means that the 'unit' of command (individual regiment, or brigade, or division) features the ability. For example, sharpshooters affect only their regiment unless they form a division, then their ability is for the entire division.
Group:
This means that the entire stack experiences this bonus. If an element or a leader, all they have to be is in the stack to experience this benefit.
However, how does this affect a leader which has a 'group' ability and they are commanding a division?
Do these abilities transfer to other divisions in the stack, or just their division?
Leader:
This means that the entire stack experiences this bonus IF the leader is in command of the stack (i.e., of top seniority). I believe it works similar to that of Group abilities, except with the specific criteria that the leader must be in command for the benefit to be experienced.
However, I have a similar question as to above, does a leader who is command of a division have these abilities (if possible) affect the division even if they are not in command of the stack? (i.e., if a leader has an ability that affects cohesion with the designation of - leader - will it affect the units in their division even if they are not stack commander? And if the end up being both stack and division commander, do all units in the stack recieve the bonus?).