New in 6.4-4
For applications/renderers that do not support using all cores while rendering (or changing that behavior, eg AfterEffects or 3dsMax), this section is not visible.
Render on all Cores
Checking this box means that once this job is assigned to a machine, no more jobs or instances will be assigned to the same machine until this job is complete. You might choose this if your know your render job is very memory intensive and shouldn't run alongside other jobs.
Min Free Slots
This is the number of slots that must be available on a worker in order for the worker to accept the job. For example, if you choose '1', a worker with 7 out of 8 cores already in use will still accept this job. However, if you were to choose '2', the same machine under the same circumstances would not accept the job.