Tormy emailed me about some tags so I figured I'd explain them here:
> USE_ANY_PET_RACE
assuming it passes other checks, any creature in their list of usables (from common domestic if they have that or from the surrounding 7x7 or so of squares and map features in those squares) which has PET or PET_EXOTIC will be available as a pet (next time, it'll let them use mounts/pullers/pack animals of any kind as well, but I'm not sure this matters in the stock raws). This notion of the initial usable creature list, which then gets pared down or otherwise considered, applies below as well. All common domestic and equipment creatures are also added to the initial list.
> USE_CAVE_ANIMALS
If they don't have it, creatures with exclusively subterranean biomes are skipped.
> USE_EVIL_ANIMALS
Don't have it -> EVIL creatures skipped
> USE_EVIL_PLANTS
> USE_EVIL_WOOD
As EVIL creatures for all uses of plant/wood
> USE_GOOD_ANIMALS
> USE_GOOD_PLANTS
> USE_GOOD_WOOD
As EVIL.
> USE_MISC_PROCESSED_WOOD_PRODUCTS
If wood is available locally, and you have permitted the relevant professions in the def, controls availability of lye, charcoal, potash and pearlash.
> COMMON_DOMESTIC_MOUNT
> COMMON_DOMESTIC_PACK
> COMMON_DOMESTIC_PET
> COMMON_DOMESTIC_PULL
If a creature has PET and the relevant COMMON_DOMESTIC_x tag (or COMMON_DOMESTIC for pets), it will be allowed if it passes the other checks.
> IMPROVED_BOWS
Adds decorations to weapons generated for bowman and master bowman. An elf hack.
> EQUIPMENT_IMPROVEMENTS
Adds decorations to equipment based on the level of the generated unit. Also improves item quality.
> LOW_SKILL
No longer used. Formerly used to keep kobolds under control.