This old command, dating back from BOA1, is useful to select the biggest stack on map you have.
It has been improved with 2 options:
SelMostNumGroup = [;NoSelGroup] [;Area AreaUID] [;CPCount] [;EvenFixed] [;NAVAL]
EvenFixed means that even fixed stacks can be selected (default no)
NAVAL means you want to select fleets (default land)
Lone Leaders are never counted in any case as a valid unit.