Log Shipping and purpose of the .tuf file

We have setup Log shipping on our production Database between two different
Data Centres in different cities. The process works well. All I want to know,
is what is the purpose of the .tuf file?>> All I want to know, is what is the purpose of the .tuf file?
.tuf stands for Transaction Undo File. It is actually used while restoring
the transaction logs with the STANDBY option. Generally when you do a
transaction log restore, you can specify this file along with your RESTORE
LOG statement, for syntax details, refer to SQL Server Books Online.
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Anith