Several of the clock routing examples have a Destination Terminal of "BoardClk". I can't find any documentation on what this is or where it goes. I've attached one of the examples, "Route Clock.vi", which can also be found under Example Finder > Hardware Input and Output > Timing and Synchronization > Signal-based > Route Clock.vi.
The specific hardware I'm experimenting with is the PXIe-6674T. I've looked in the manuals for the PXIe-6674T and the PXIe-1082 chassis. Google has no results.