Binary 100 = Randomize unit loading position.
In dungeon and skirmish maps, when the event code [2B21] is called before a LOAD instruction, the parameter of this code is used to configure the overall probability that units marked with flag 100b will have their starting position modified, based on their initial coords and their ability to cross the surrounding terrain.
Without calling event [2B21], or calling with no parameters, this flag has no effect (it will be cleared when loading the unit data into the EventUnitDataBuffer). This is why it was thought to be unused.
Investigation of ASM
Event2B_ stores the parameter of [2B21] to MapEventEngine+4E.
This value is read at FBF0.
If this value is not set, “Randomize unit loading position” flag is cleared when copying the unit to the 0203efb8 gEventUnitDataBuffer (clearing occurs at FA5C based on parameter in r10).
If the flag still exists in the unit data buffer at this point, a helper function at 0807a014 MoveUnit_ will construct a list of spaces within the unit’s starting movement range, choose one based on RNG result, and place the unit at those new coordinates.
Sources:
Stan Event Doc (event code 2B)
Results of my own research, documented here.