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The Job Slots (or Process Slots) for a Worker determine how many processes a given Worker can run at a given time. The Job Slots have no direct relation to the number of processor cores on a given machine. It is often desirable when using a multithreaded multi-threaded renderer like Maya to have the Worker run 1 instance (or process) at a time. That process will in turn utilize all processor cores for the rendering. If wanting , using all the cores for that instance. On the other hand, if we want to run many single-threaded processes that each require little a few resources, one you may want to set the Worker to have 4 or 8 Job Slots available.
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Jobs can be submitted with restrictions, such as "only run on OS X". Workers can also have restrictions on the number or kinds of jobs they will run; for example, a machine may be restricted to running only After Effects jobs.
Qube! also has the concept of clusters. A cluster is a group of machines that is given a name. Clusters have a hierarchy, so that if a job cannot be run in one cluster, it may be able to run in a parent cluster. The most common example of this is show/sequence/shot. If the appropriate resources are not available in the "shot" cluster, the job may be assigned to the "sequence" cluster, and so on.
Workers have resources, such as CPUs and memory, that jobs may specify as requirements. The facility may also have resources, such as software licenses, that jobs my specify as requirements. The Supervisor looks for Workers that have those particular resources, and then reserves slots on them for the job(s) it wants to assign.
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