FlexLetters are a pre-approval letter that your real estate agent can use over and over again. Your lender sets the maximum approved price, and sends the letter to you and your real estate agent.  A copy of this letter, with the max approved amount, appears in your Document Vault.
Your real estate agent can reissue the letter with a different price (lower than the maximum) to match the home you are bidding on. When they do this, another flex letter appears in your documents.