Some companies operate what are known as "draw-down" contracts. What this generally means is that although terms are agreed and a contracts are signed etc., a candidate is not engaged on a full time basis. Rather the parties have a mutual agreement that they may work together sometime in the future for an agreed rate, and on a mutually convenient times.
Its easier to explain by example. So, Stephen has agreed a draw-down contract with CPL for 6 months, it will mean that he will stay in touch with CPL and if he comes available during a time when CPL need him, the parties will engage.
The benefits of such a contract are as follows:
For the contractor, it can "shore up" time between more traditional contracts.
For the employer, it can mean that they do not have to pay the full cost of having a contractor during quiet times.
Its efficient for both parties financially - a contractor can ask for a premium rate for such a contract, and even though the contractor is doing so, it is still more cost effective for the employer because they are not billing 100% of the time.