If opponent doubles a major suit transfer,

XX = business

accept shows fit, + stopper in opponents suit

pass shows no stopper in opponents suit and or doubleton in major

xx by responder = retransfer (in ♥ auction responder must xx with 4441 etc)

If responder bids new suit it is forcing.

After XX retransfer then if responder bids new suit it is non forcing

If responder accepts his own transfer it is invitational

If responder jumps and accepts his own transfer it is forcing