From the look of it, I think, txHCTrunkFrames should be the number of trunk frames ( isl or 802.1q) transmitted on this particular port, and rxHCTrunkFrames should be the number of trunk frames ( isl or 802.1q) received on this particular port.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus