A Custom CDR might be many different things. Whatever your definition a Custom CDR will be created for the purpose of raising interest in the CD product you have to offer. Often when someone talks of a Custom CDR they may be thinking about a CDR that has been cut to a shape such as a Shaped CDR. Like Shaped CDR, Custom CDRs can be made into any shape such as a skull, a strawberry or a birthday cake. The top of the Custom CDR will usually be printed in the style of the CD shape.
Other people might consider a Custom CDR to be any CD that they have produced and not bought. Therefore a selection of audio tracks compiled at home and burnt onto a CD might be considered to be a Custom CDR.
A marketing executive might commission a creative agency to build his company a Custom CDR to help meet the company's marketing requirements.