You use the Compact Outline dialog box to compact aggregate storage outlines and, thus, to reduce the size of outline files.
If you want to work during the compaction process, select “Compact outline in the background.” During a background process, you can exit the console but cannot shut down Essbase Administration Server. You can view the status of the process in the Background Process Status window.
Compacting Aggregate Storage Outline Files
alter database (MaxL) in the Oracle Essbase Technical Reference