I think that you are referring to Online vs. Offline operations (From Books Online):

DBCC INDEXDEFRAG is an online operation. While this operation is running, the underlying table is available to users of the database.

DBCC DBREINDEX is an offline operation. While this operation is running, the underlying table is unavailable to users of the database.

AFAIK, there is no way to specify a particular user/spid has lower priority than another. This is possible with Windows processes, but not SQL Server users.