The Nissan Path

Hello, I just bought a used 2008 "S" Pathfinder. I noticed there is a in-line fuse holder coming off the positive battery terminal and goes into the fuse-able link. It has no fuse in it and could not find it the owners manual.
Is this standard?
What could this be for?
What size amp fuse should go in there?

Thanks

A pic would help. Most likely something aftermarket like an alarm, amp, or remote starter.

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