
Over a 1M opening, I play 2♣ as showing (a) gameforce with 5+ clubs, (b) balanced gameforce or © invitational with 3-card support. This allows me to have my 2♦ response promise 5 diamonds and facilitates making 1NT nonforcing while still letting me use 1M-3M as a mixed or preemptive raise.
Over 2♣, opener will usually bid 2♦. Opener only breaks from this relay if he (a) has significant extra shape (i.e. not 5-4) and (b) is willing to accept the invitation. 1M-2♣-2NT shows 6-4 with a minor, and 1M-2♣-3m shows a 5-card minor. Over these relay breaks, 4M by responder shows the merely invitational hand, with GF hands bid naturally.
Here are the continuations over the relay. (Note: M is the major we opened and oM is the other major)
1M-2♣-2♦-...
...-2M Invite, 3-card fit, nonforcing
...-2oM GF, 2-4 clubs, 3-card fit
...-2oM-3x Natural, 4+ suit
...-2oM-3M+1 No slam interest
...-2NT GF, 2-4 clubs, 2-card fit, 12-14 or 18+
...-2NT-3x Natural, 4+ suit
...-2NT-3M Sets trump (after this 12-14 hands should generally bid frivolous 3M+1/3NT)
...-3♣ 5+ clubs, no 3-card fit
...-3♣-3♦ 4+ cards in other major
...-3♣-3M 6+ card suit
...-3♣-3oM club fit
...-3♣-4♣ 4+ diamonds
...-3♦ 5+ clubs, 3-card fit, short diamonds
...-3M 5-6 clubs, 3-card fit, no shortness
...-3oM 5+ clubs, 3-card fit, short in other major
...-3NT Balanced, 2-card fit, 15-17
...-3NT-4x Cuebid setting opener's major as trumps
...-3NT-4NT Quantitative
...-3NT-5m To play