Thanks Brendt. Here is what I found

Transaction undo file

File containing information regarding any modifications that were made as part of incomplete transactions at the time the backup was performed. A transaction undo file is required if a database is loaded in read-only state. In this state, further transaction log backups may be applied.

http://www.microsoft.com/technet/prodtechnol/sql/2000/maintain/sqlbackuprest.mspx