Final answer:
A blanket mortgage with a provision for releasing a single parcel upon a specified payment contains a release clause, which is beneficial in real estate development for selling individual lots.
Step-by-step explanation:
A blanket mortgage that allows for the release of a single parcel upon payment of a specified sum contains a release clause. This clause enables a portion of the property to be released from the mortgage lien after the borrower has paid a certain amount, as stipulated by the terms of the mortgage. It is particularly useful in real estate development, where the developer wishes to sell individual lots and use the proceeds to pay off portions of the loan.