A sequel is a continuation while a spin-off is a separate story set in the same world, and with some or most of the characters from the main story. Slice of life AUs are spin-offs but a slice of life afterstory is a sequel.
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I've looked at them but does it help with gaining readers? I know good reviews can help but does it give any form of exposure and/or put the reviews on Amazon and goodreads? I tried with onlinebookclub and the reviews didn't leave the site.