[Edi]

Unique identifiers, unit numbers (unr) are recycled, so your commander can get almost anything from among the free ones. When you recruit units in bulk, they get assigned from free blocks and each recruitment gives as consecutive numbers as possible to the units, meaning they will also appear in that order in the squad you assign them to.

If you want to do specific types of mixed unit formations, recruit the in the order you want them (e.g. alternating shield maiden/steel maiden to get a little bit more durability without sacrificing to much offensive power from the squad as EA Ulm)