When creating a new torrent the comments box has a source key field what is that for?

It's a field added to the info dictionary of a private torrent to make its hash unique even if content (files) are the same, akin to salt in cryptography. Used by some private trackers to let the same client seed multiple copies of data as different torrents to different trackers. Matching hashes have probably bothered seedbox owners and other massive hoarders too often.

Source key is a string of salt to generate unique hash. Useful in private torrenting or just to mark your torrents.