Frequently, I find a story with an interesting blurb, start it, and later decide I don't like it. Then months or years later I find the same story again but don't remember I already tried it.
By putting it on the Reject List, FFF remembers for me that I've already tried that story and rejected it--and why. You can maintain your own list of quick-select reject reasons as well as enter specific text for any as you like. It's also very useful for avoiding stories I don't like from an author that I'm following.
Reject List is one of the most useful features in FFF, IMHO. I have 1900+ entries in my reject list.
When you invoke the 'Reject Selected Books' action, at the bottom of the dialog there's a checkbox to also delete the books ('Delete Books (including books without FanFiction URLs)'). The 'Delete on Reject by Default' settings checkbox is simply the default value for the delete books checkbox on the Rejects dialog.
Personally, I remapped the hotkey 'Del' to 'Reject Selected Books' and changed the normal delete function to Ctrl-Del.
And yes, you could put books you don't want to update or overwrite by accident in the Reject List. Just be careful not to delete them. And give a 'reject reason' that makes sense to you.
My personal Reject Reasons list, for the curious:
Spoiler:
- Cross with something I don't care about
- Fandom I don't care about.
- Dup from another site
- Included in Anthology book
- Series I'm not following
- Random linked from an SB/SV post.
- Just poor
- Just bad
- Crack
- Sat on the 000 list for months.
- Appears abandoned
'000' is how I mark new/to-read stories.
This page is a bit dated, but I believe is still reasonable accurate.