What are the contracts which cannot be specifically enforced under the Specific Relief Act, 1963?

According to Section 14 Specific Relief Act, 1963 the following contracts cannot be specifically enforced, namely:- where a party to the contract has obtained substituted performance of contract in accordance with the provisions of section 20 of the Act; a contract, the performance of which involves the performance of a continuous duty which the court cannot supervise; a contract which is so dependent on the personal qualifications of the parties that the court cannot enforce specific performance of its material terms; and a contract which is in its nature determinable.