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The type descriptions contain much more than that, for instance valid range for integers, field layout for structures and so on. Also privacy information.

Ok thanks for the clarification

My guess is that it is an object oriented system. The data's type is a pointer to the address that defines the type. Which could be anywhere in the system.

This is also a security feature. If you find a way to randomly change the data's type, you're unlikely to successfully change it to another type.

I don't know if this machine supported it, but it could allow you to have a system-wide unique type for this-struct-in-this-thread-in-this-process, with strong type checking all the way through the compiler into run time. Which would be pretty cool.

GUIDs for types.

No idea, but consider that it could be a enum + bitfield rather than strictly an enum.

The other option is to use those 64bits to double the total bandwidth in the "Traditional" page-table system.

All this extra complexity and bus width doesn't come for free, after all, there's opportunity cost.