What is the .srr extension used for when coupled with an .avi file?

These are related to "Scene releases". SRR files are created by ReScene.

FAQ

What is this .srr file?

The .srr file is the format used by ReScene. It contains backups of all parts of the RAR files that are not the actual archived data. It can also store any other miscellaneous release files (e.g. SFV, NFO, etc). It is also an extensible format that will allow for other uses in the future.

The old site with info and downloads is down. (rescene.info, rescene.com)

Now you can find all relevant information here: http://rescene.wikidot.com/

A database of SRR files can be found here: http://www.srrdb.com/