A user has been assigned with five Operating Units in his or her security profile. The user transacts In three of those Operating Units, the remaining two are used occasionally. The user can limit the list of values available to the three Operating Units by. 

setting the preferred Operating Units in user preferences 

setting the values for the MO: Operating Unit at the user level profile option 

setting the values for the MO: Default Operating Units at the user level profile option 

requesting the system administrator, to update the Operating Units in the security profile 

requesting the system administrator to set up MO: Default Operating Unit for the responsibility