FUB must stand for something like "Functional Unit Block" because P54C refers to the processor core (P54C is a specific version of the Pentium core, though the actual core in the Xeon Phi has been heavily upgraded from the original P54C), CRI refers to the "Cache-Ring-Interface", and VPU refers to the "Vector-Processing-Unit".
In the Xeon Phi performance counters, the UMASK field actually specifies the functional unit for which the event is requested, with 0x00 referring to the core (P54C), 0x10 referring to the CRI, and 0x20 referring to the VPU.
This was confusing to me at first because because on other processors the UMASK is almost always used to modify the specific details of what is measured by an Event Select code, rather than actually specifying the Unit on which the measurements should occur. On Xeon Phi the UMASK field is used in a way that is much closer to what you would expect a "Unit Mask" to mean -- it specifies the "Unit" for which you want the measurements to be taken.
