Use of the Alias Database is discouraged. Use the aliases file instead, especially since it can be dynamically reloaded using the imsimta reload command.
The MTA uses the information in the directory and creates the alias database. The alias database is consulted once each time the regular alias files is consulted. However, the alias database is checked before the regular alias file is used. In effect, the database acts as a sort of address rewriter that is invoked prior to using the alias file.
The format of the database itself is private. Do not try to edit the database directly. Make all required changes in the directory.