From Law wiki, the wiki for law research

The transfer (typically by sale) of an interest in land or property. The interest need not be absolute; for example, Leasehold interests can be assigned. An assignment is more than just a sale: sales are governed by the law of contract, while assignments may be binding on purchasers who were not parties to the original agreement. For example, if freehold owner F1 grants a lease of his property to L1, and L1 assigns his lease to L2, then F1 may have obligations to L2 even though these people were never themselves parties to a contract.