In the Acol Bridge Bidding System, Red Suit Transfers (RST) are only used after your partner has opened 1NT.
The idea is to transfer the play of a major suit from the responder to the opener.
To transfer the responder must have at least 5 cards in a major suit.
If the responder has 5 hearts, she bids 2 diamonds, if 5 spades she bids 2 hearts.
It is likely that the opener will have 3 cards in any suit so there will be a fit.
If the opener has 4 in the transferred suit she bids at the 3 level (a super transfer).
Transfers are used if the responder has a weak hand and wants to avoid NT or has a strong hand and wants to give a choice to the opener of game in NT or a in a major.
If responder has a hand between 7 and 10 points she should leave in bidding in 1NT.
Unless invited to make a choice the opener must make no bid other than the bid to accept the transfer.