Reference : Note 745420.1 How To Setup and Verify eBS MO/MOAC, Defaulting Rules and MO: Security Profile with Channel Revenue (Trade) Management Example

If the profile option MO: Security Profile is not set, then MO: Operating Unit value is used as the default Operating Unit even if MO: Default Operating Unit profile is set to a different value.
If the profile option MO: Security Profile is set and gives access to only one Operating Unit, the default Operating Unit will return this value even if MO: Default Operating Unit is set to a different value.
If the profile option MO: Security Profile is set and gives access to multiple Operating Units

If the profile value MO: Default Operating Unit is set, it is validated against the list of Operating Units in MO: Security Profile
If the Operating Unit is included in the security profile then it is returned as the default value.
Else there is no defaulted Operating Unit
If the Profile Option MO: Default Operating Unit is not set, then there is zero (no) default Operating Unit.