The reindex command reindexes all of the documents already in the Database.
It can be used to apply a new Database policy to existing documents. In this case, any previous document policy is discarded.
It's also a method how to repair corrupted index.
"Background" Reindex command switches to new policy only when Reindex is done. When database is in reindex process, existing policy is valid. To turn on "background" reindex, parameter <in_background>yes</in_background> should be defined within Reindex command.