From Law wiki, the wiki for law research

A contract to convey an interest in land, or a contract creating the right to take an InterestInLand. If A makes a contract with B to sell him his house, then an estate contract arises in B's favour. B now has an equitable interest (under a ConstructiveTrust) in A's land. The question then arises what would happen if A did not transfer the estate to B with the appropriate formalities, and later sells the land again to someone else? As an equitable interest, B's right must be duly protected if it is to be enforceable against the later purchaser. This means, in practice, the registration of a class C(iv)land charge if the land is unregistered, or a Notice if it is registered.

In a straightforward conveyancing transaction, where the contract is specifically to transfer a registered legal estate, then the application for apriority search usually makes it unecessary to enter a specific notice of the contract.

A contract to convey is not the only estate contract that is recognized. Other examples include an option to purchase and, with a Lease, and OptionToRenew.