Responder Has Less Than 12 Points
After 1 club opening anything besides 1 diamond is non-forcing and shows less than 12 points.
All these bids should be alerted:
1C 1H natural in hearts with less than 12 points
1C 1S natural in spades with less than 12 points
1C 1NT responder may have a diamond suit
1C 2D shows 6+ diamonds with 6-9 points
Note, 2 diamonds over 1 club usually is pre-emptive. It was felt the 6-9 point hand was a better use of this bid.
1C 3D 6+ diamonds, 10-11 points
Inverted minor responses:
1C 2C 5+ clubs, no 4 card major 10-11 points
1C 3C 5+ clubs, no 4 card major, 6-9 points