A lot of renderer or plugins do not have unlimited render licenses.
Therefore you can tell RR how many render licenses you have.
See rrConfig, tab "Render Lic."
In addition to counting the licenses that RR uses, RR is able to reduce the number of licenses temporally if they are used by an artist.
If a client returns a job with an error and a license error message was found in the log file, then the server reduces the render licenses used in this job for 1 hour.