.TUF file is the Transaction Undo File, which is created when performing log shipping to a server in Standby mode.

When the database is in Standby mode the database RECOVERY is done when the log is restored, and this mode also creates a file on the destination server. TUF extension which is the transaction undo file.

This file contains information on all the modifications performed at the time backup is taken.

The file plays an important role in the Standby mode… the reason is very obvious while restoring the log backup all UNCOMMITTED transactions are recorded to the undo file with only COMMITTED transactions written to disk which enables the users to read the database. So when we restore the next transaction log backup; the SQL server will fetch all the uncommitted transactions from undo file and CHECK with the new transaction log backup whether committed or not.

If found to be committed the transactions will be written to disk else it will be stored in undoing file until it gets committed or rolled back.

If .tuf file is GOT deleted there is no way to repair log shipping except reconfiguring it from scratch.