Allied Telesis AT-SBx81CFC960 - Manual (Page 272)

Chapter 14: VCStack Plus Overview
272 Section III: Building a Stack with VCStack Plus
VCStack Plus feature.
 The stacking transceivers must be connected to other stacking
transceivers. You may not use them as regular networking ports.
 The VCStack Plus feature must be activated on the two switches
of the stack for the SFP+ slots and stacking transceivers to forward
stack traffic.
 You may use the SFP+ slots on the controller card with standard
SFP+ transceivers when the VCStack Plus feature is disabled on
the chassis.