There are quite a few different ways to play over an 18-19 point 2NT rebid, such as Wolff Sign Off, New Minor Forcing and Checkback Stayman. There is no real standard among intermediate players. There is however a standard among expert players. Expert players play transfers after an 18-19 2NT rebid by opener.
By playing transfers you let the responder tell about their hand, rather than ask about opener. By playing this way, you give responder the flexibility to sign off, find the right game or explore slam while also keeping the declarer's hand better hidden. This lesson will teach you the entire system of transfers after 2N rebid.