it can be a crap shoot on designation. TDs can also be wheeled as in the Italian 8x8 CENTAURO, a mean looking bastard with a 105mm. the PLA had a 6x6 TD too. Most likely it’s designated by its perceived deployment especially gun vehicles. Then there is the whole ww2 assault gun (STG-III, HETZER, SU-85) which were more AT and assault gun (STuH-42, SU-122, SU-152) which were more direct fire artillery and US (M-10, M-18, M-36) which were designated TDs. add to yhe mix the Wehrmacht “JAGD” (JAGDPANTHER, JAGDTIGER) vehicles which were TDs. anything with an ATGM is an AFV killer.
if you really want to be confused look at naval designations for destroyers, destroyer escorts, frigates, and corvettes.
These vehicles could be used as assault guns in infantry direct support roles.