Formula 1 - Bikes that complied with FIM TT Formula 1 rules, basically 1000cc four strokes in special chassis and up to 500cc 2 strokes in special chassis. The engines had to be based on current production road engines but the chassis etc was free choice.
Formula 2 and Formula 3 same philosophy as F1
F2 - 4 strokes up tp 600cc, 2 strokes up to 350cc (many of the 2 strokes were lightly modified TZ Yamahas using LC cylinders)
F3 - 4 strokes up to 400cc and two strokes up to 250cc (viz 250 TZ instead of 350 TZ). Another successful F3 was the 350cc single cylinder Aermacchi Ala'Doro which as it was based on a road bike qualified without any modifications.
Note - the early 73-80 TZ were based on the RD road bikes with similar engine/frame configurations but upgraded for racing and water cooled. The liquid cooled LC Yamaha introduced in the late 70s meant that the F2, F3 bikes could run TZised LC motors including dry clutches etc.
Senior - Up to 1000cc
Senior - open up to 1000cc (big 4 cylinder 4 strokes), TZ750 2 strokes, RG500 two strokes and any other combination
Junior 250cc - Two strokes (mainly) up to 250cc, but when the 350s became obsolescent (350 Yamaha TZ etc) they were allowed alongside the 250s (after '81 I think)
Senior Classic - 500 cc racing machines prior to 1968 (I think that was the cutoff date) - 2 strokes nota allowed
Junior Classic - 350 cc racing machines prior to 1968 (I think that was the cutoff date)
Lightweight Classic - 250 cc racing machines prior to 1972 (I think that was the cutoff date)
The cuttoff dates exactly for the Classic I am not sure of (never raced this type of bike apart from a brief stint on a 350 Aermachi Mettise in 1972)
Some info anyway.