[Fate]

It is related -roughly- to the order you recruit them in. The Unit ID (like many Identifiers) is simply a number assigned to each unit to keep track of them.

Units are listed on the map according to their ID. I think it is safe to assume each new unit is assigned the next vacant ID (but remember old IDs can be vacated when old units die, and so new units may appear in the middle rather than the end). Once an ID is assigned, it cannot be changed.

Rituals will be randomly cast in either ID order or reverse ID order.
Battle spells will always be cast in ID order.