What is a Trace frag? Where do we use it?

Temporary setting of specific server characteristics is done by trace tags. DBCC TRACEON is the command to set the trace flags. Once activated, trace flag will be in effect until the server is restarted. Trace frags are frequently used for diagnosing performance issues. For example, the trace flag 3205 is used for disabling hard compression for tape drives, when an instance of SQL Server starts.