These days more and more DJs are using digital equipment rather than physical CDs or vinyl records. It is a legal requirement for DJs who "format shift" digital music (eg. copy from one medium to another or make multiple backup copies of files) to hold a ProDub Licence, administered by PRS for Music on behalf of the rights owners.
It is a requirement of a venue's own public performance licence that they validate the ProDub status of digital DJs playing there and we are now experiencing more and more venues asking for evidence. It's entirely feasible that a DJ or entertainer without ProDub may be denied access on the day of your event.
The correct licencing of music is a complex area. For example, most private parties held in venues with no public access are exempt from public performance licencing but visiting DJs working for gain must adhere to ProDub. If you'd like to discuss this in more detail with us, we'll be happy to try to explain things to you. Alternatively, visit PRS for Music or the specific ProDub website.