The subject of a contract in law is defined as the element which the party that owes money commits to do. The subject could be performance of an action, refraining from the performance of an action or a transfer of the right to some property.
Source: Kharofa, Ala’ Eddin. Transactions in Islamic Law. 2nd ed. Kuala Lumpur: A.S. Noordeen, 2000. 22.