Not sure why they chose T7 to stand for "Twin Jet" aircraft. There are some things that just can't be explained with the IATA and ICAO.
Some are clever, some are not, and some are just leftovers. Comair (OH) is based in Cincinnati, OHio metro area, so that makes sense as a clever one. Another was ACA/Independence (BR), which recalled to ACA's callsign Blue Ridge, since they were based in the Blue Ridge Mountains.
Skywest is OO and ASA is EV, I don't understand those. And Freedom and Frontier are F8 and F9, which seem to be just leftovers.
The legacy carriers have pretty normal IATA codes though: Delta (DL), American (AA), United (UA), USAirways (US...I assume US, and not HP)