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Jobs can be submitted with requirements, such as "only run on OS X" or "the processor must be at least 3GHz" . Requirements are statements about machine properties that must be available. Workers can also have requirements; for example, a machine may be required to run only one After Effects job at a time. Properties are different from resources because you can't "run out" of properties. For example, a machine is running Windows no matter how many jobs land on it.
Workers have consumable resources, such as CPUs and memory, that must be reserved by jobs. The facility may also have resources, such as software licenses, that must also be reserved. The Supervisor looks for Workers that have those particular resources, and then reserves slots on them for the job(s) it wants to assign. Resources must be reserved because jobs use them up, and so you can run out of them.
It may help to think of this in terms of restaurants. There are many restaurants in the city. You might require a restaurant that has 20 tables in order to host a wedding reception - any restaurant with 20 tables will do. On the other hand, let’s say a restaurant has 40 seats. Those seats are resources, and you could reserve 4 of those seats for your party. While you are using them, they aren’t available to anyone else, which means there are only 36 seats available. As soon as you leave, those seats become available and the restaurant can again advertise that it has 40 seats available. So you require properties (at least 20 tables), but you reserve resources (4 seats).
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