Class here works about philology in the sense of (especially historical) linguistics. Class historical descriptions of particular languages or language groups at the number for the language or group, with the addition of -112 (from 81-112)
The -11/-26 and `1/`443 special auxiliaries may be used with 81 itself to denote the subject matter of linguistics in general, and also in combination with the subdivisions of 811 for linguistic aspects of particular languages or language groups