I believe now that the numbers refer not to the quality of the rip but the degree of compression.
It is. Analogous to the similar setting available for ZIP compression. It changes the balance between resulting file size and decode complexity. It has no impact whatsoever on sound quality. It is a setting for compression "strength" (the "quality" of the compression algorithm).
There is no reason to change it. It doesn't help with anything and makes the resulting files larger (or marginally smaller). But, if you insist, the option can be found under:
Options > Encoding > Encoder Settings.
No matter what this is set to, the resulting output is identical. If you "lower" the compression strength/quality, then the flac files it creates will be larger and will take a millionth of a second less time for a modern CPU to decode. If you "raise" the compression quality, then the files will be larger and they'll take a millionth of a second longer to decode. Like the similar setting for making ZIP files, this is mostly a hold-over from times when CPUs were
far less powerful, and when some weak CPUs might have had difficulty decoding a complex FLAC file. This is no longer an issue for any CPU made in years and years.