Whatever the physical channel (single-mode fibre, multi-mode fibre or free space), the quantum channel interface should pass light into the module's components in one lateral optical mode only, so that attacks exploiting multi-mode behaviour of the module are excluded; for that reason the series considers only single-mode optical fibre as the quantum channel interface.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.