Three Club Response to 1D opening

This is a conventional response used in some 2/1 systems to show exactly 6-7 clubs and game invitational values (10-12 HCP). Otherwise, responder might bid 2C followed by 3C where the rebid of a suit cancels the game force. Since responder will bid 3C to show invitational values, the 2C followed by the 3C rebid is game-forcing and suggests a possible slam in clubs.