Just like the first reply, SCO is Panasonic nomenclature used to describe a button strictly defined as one specific CO port, not a non-specific one that may be in a particular group. (That would be a GCO button.)
If the CO port defined under the SCO button is in use, the button will light, if it's ringing (and not defined as a DIL1:1 to another particular station) it will flash.
Pat MacKinnon
Omni telecommunications Inc.,
London, Ontario, Canada
e: [ pat & omnitel , ca ]