Subcategories (a.k.a. slims)

Subcategories are useful for categorizing and (de)selecting groups of classes. Examples:

'internal' vs 'external' - If we're combing species descriptions (especially older ones) for EQ statements that refer to head phenotypes we might want to exclude internal characters from our logical inferences. E.g., a statement like "head black and rugose" implies that the parts of the head (face, gena, vertex, etc.) are each black and rugose, but the author was probably not referring to internal structures (tentorium, mandibular muscles, etc.) that are also technically part of the head.
'public' vs. 'private'

These could likely be created as tags in mx.