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Oh, so you're actually interested in this stuff. Cool!
The IF master is what updates GUID's and SID's in a multi-domain environment so that objects from the domains can reference each other. So it has to dynamically know what objects exist in it's domain only. Since a DC (domain controller) that is also a GC (global catalog) always knows about all objects in _all_ domains (known as the forest of domains), the IF master and GC are incompatible roles.
MS does a better job of explaining this in knowledge base article Q197132, and also gives a description of all the roles the DC's can hold. Enjoy!
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