a non-jump reverse by opener is forcing for one round (for example, 1C - 1S - 2H). then
responder rebid his major with 5(+) cards. this is forcing one round and the only way to show a 5+ cards (for example, 1C - 1S - 2H - 2S)
the cheapest announce between 4th suit and 2NT is artificial, weak. opener make the most economic call with a normal reverse (~ 15-17) or anything else (natural) with a strong reverse (~ 18+ HCP)
any other announce is natural and creates GF