Limit Per Membership Plan restricts a coupon to customers on one membership plan. Leave it at Select A Membership Plan for no restriction.

When a plan is selected:

• A customer who types the code and is not on that plan (or is not logged in) sees This coupon is not valid and the code is not added.
• If the coupon has Auto Apply Coupon to Cart turned on, it is silently skipped (no message shown) for shoppers who aren't on the plan — it only actually gets added to the cart for members.
• Show as Suggested Coupon works differently: the suggested-coupon list shown at checkout is not filtered by membership plan (and isn't even filtered by whether the coupon is still active or unexpired) — every shopper sees every coupon flagged as suggested. Clicking one runs it through the normal check, so a non-member who clicks a plan-restricted suggested coupon still gets This coupon is not valid.
• The plan check runs together with the coupon's other rules (Limit Per User, first-checkout-only, minimum order, allowed modules, expiry, remaining uses, can-be-used-with-other-coupons). All must pass.

Only one plan can be chosen per coupon; to cover several plans create one coupon per plan.