Yes, rate lock as it applies to this scenario is the lenders agreement with the borrower. The LE has fields to disclose whether or not the rate is locked with the borrower. Once you send them an LE indicating their rate is locked and an expiration date of that lock, you are locked into that rate and credit.
Unless one of two things occur; 1. the rate lock expires, 2. the borrower is declined for eligibility and counter offered with a new offer.
You can also add discount to buy down the rate as long as it is bonafide (one quart point rate reduction for every 1 point of discount charged.)