UK Radio Authority
parlance for a Restricted Service Licence.
An RSL is a temporary licence for a station to go on the air, typically for 28 days, on a low-power transmitter, and no more than once in a six month period. They are normally used for community groups, student radio stations, or commercial radio stations trialling services. They may also run alongside large events, such as music festivals or sports tournaments.
Long term RSLs can also be granted, for example some football clubs will broadcast match commentary around their ground. Some hospital radio stations broadcast on induction loop systems and require such a licence. Low Power FM and Low Power AM also fall under this kind of licence.
More information can be obtained from The Radio Authority at http://www.radioauthority.org.uk/rsl/