This convention is used after 2NT (showing a strong Balanced hand) to find a 4-4 / 5-4 fit.
3C ask pd to start bidding 4+ card at the 3-level.
- 3C : Ask pd to bid 4 card+
3D (4D+) - 3H (4H)
3S (4S) - 3NT (No fit)
2NT - 3C
3NT : No 4D/4H/4S so therefore 4C+