I'm wondering about SELECT_SYMBOL. ...but how often exactly?
SELECT_SYMBOL means that a name based on the chosen category (CIV, SITE, OTHER, etc) must use a word from that symbol once. If it's a "The X of Y", then the symbol will always be used as "The X" noun, while compound names can put the symbol in either slot. First names (which are linked to the OTHER category alongside site compound names, usually hit with REMAINING after defining everything else) either use all nouns or nouns from the selected symbol, always respecting culls.
SUBSELECT_SYMBOL works the same for compound names, but will be used for "of Y" or as an adjective in "The Y X". It will also not be used to create first names.
Ie., remove the AQUATIC words from the SUBSELECT?
Culling only targets SELECT_SYMBOL. If you want to further limit a subsymbol, all symbols are raw-defined in language files, so feel free to make your own hand-crafted symbol.
I tried it, and saw creatures with almost identical names, and it was kind of lame.
This is the v47 modding thread, so specific to that version there's bugs with the SUBSELECT_SYMBOL tag - I've had issues where practically every orc was named "Gaisk", and so on with modded civs I gave subsymbols to. I don't know the precise logic, since it all works well and good as I described in v50 and all I took away from it is to not use SUBSELECT_SYMBOL on OTHER and/or REMAINING.