The Taco Club and transfer responses
In Modern American, we have tightened our requirements for all the opening 1-level bids. We are playing 5-card majors, so opening one of a major requires at least 5 cards in that suit. Our 1NT opens are balanced hands with 15-17 HCP. They might have a 5-card major (since we play Puppet Stayman over 1NT, we don't worry about missing 5-3 game fits in the major). More importantly, we've restricted our 1♦ opening bids with the Unbalanced 1♦. This means that we'll only open 1♦ with a 4-card diamond suit AND an unbalanced hand. If we have a 6-card diamond suit and a balanced hand, we can also open 1♦.
What we have left is that 1♣ opening can be only 1 of 2 hand types. It will be a balanced hand and too weak to open as a 1NT. That would be 12-14 HCP. It could be as few as 2 clubs (meaning it needs to be alerted) because we'll choose to open 1♣ instead of 1♦ even when we have 3-3-5-2 shape. A hand with 5 diamonds and 2 clubs is balanced and therefore won't open 1♦. Think of these hands as a weak NT opening bid. That's why we play transfer responses to the opening 1♣.
In Modern American, transfer responses make this odd case of opening a club, even with a longer diamond suit, logical and desirable. The transfer responses to 1♣ are covered in another article. Let's discuss the opening criteria of 1♣ a bit further.
Another hand that could open 1♣ is an unbalanced hand with a bidable club suit. A bidable club suit is usually 6+ cards. In a pinch, you may bid a solid 5-card suit. Such a hand would probably be unbalanced (short) in diamonds, making it impossible to open 1♦ to show an unbalanced hand. With 4 diamonds and 5 clubs (and shortness in one of the majors, you would likely choose to open 1♦ unless the clubs are solid.
Opening 1♣ when unbalanced can be any opening-strength hand: 11-19 HCP. Probably don't want to abuse bidding (and trust in partner) opening with less than 11HCP. Also, most 11HCP hands that open will have at least two Aces and qualify as two quick tricks. Hands with more than 19 HCP are too strong to open at the 1-level. 19 HCP and a singleton (or void) probably qualify to open with an artificial strong 2-level bid. See the Mile High 2♦ in the Strong, Preemptive and Competitive bidding section.
With this unbalanced 1♣ opening bid, your next rebids will tell partner you did not start with a 12-14 HCP balanced opening hand. If you cannot support their transfer-requested suit, rebidding clubs shows you have a bidable club suit and shortness (likely in their transfer-requested suit). A jump in clubs shows the same with a stronger hand (16+ HCP).