Source Endpoint & Destination Endpoint what are they and how to use them?

Been doing lots of extensive searching but can not find any any explanation as to what SE & DE are for, how to use them and why

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They are not easy to use unless you are writing your own device code, and don’t make the mistake of thinking of them as just like UDP/IP port numbers.

I believe the ‘pure’ answer in ZigBee context would be the example of a wall switch with 3 physical buttons/switches, therefore 3 sets of the ‘data clusters’ related to switches. These would be 3 distinct end-points.