A 200A service is one that can nominally supply 200A on each of the incoming ungrounded conductors. I say 'nominally' because the ratings of the transformer and the trip curve of the main OCPD come into play, but in general this means that you have service conductors with an ampacity of 200A and OCPD with a trip rating of 200A.
In the case of 120/208 3 phase 4 wire wye, a 200A service means that on any leg you can have 200A of current flow. You could have a total of 72KVA delivered as 200A on each of 3 120V line to neutral loads. You could have a total of 72 KVA delivered as 115.4A on each of 3 208V line to line loads. If you have a single load connected from phase A to phase B (or any other pair of phases), then you could supply 200A to it, but only deliver 41.6KVA. If you have a single load connected from phase A to phase B carrying 200A, and a single load connected from phase C to neutral, carrying 200A, then you will deliver 65.6KVA to the load. (As well as have 4 current carrying conductors....hmm, wonder what this says about 'full boats'
For 120/240 3 phase 4 wire delta, again, on any leg you can have 200A of current flow. You could have a total of 48KVA delivered as 200A on each of 2 120V line to neutral loads, with the 'stinger' leg unused. You could also deliver 48KVA as 200A through any 240V load connected phase to phase. Finally, with a balanced three phase load, you could deliver a total of 83.1KVA as 115.4A on each of 3 240V line to line loads.