A320
Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Procedure; Human Factors
Air Carrier X (A320) was level at 350 going into one airport; Air Carrier Y; also an A320; was at 360; landing at another. My trainee issued traffic to the Air Carrier Y; '11 o'clock one zero miles southbound Airbus 320 one thousand feet below you; expect lower passing traffic.' Air Carrier Y read back; 'roger; maintaining FL360.' Traffic was issued to Air Carrier X. About 20 seconds later; Air Carrier Y said 'traffic in sight; going direct to the fix descending to 120.' My trainee said; negative maintain FL360; issued traffic again; and told the pilot to report level at 360. But Air Carrier Y had already descended to FL357 before going back up. It's hard to say what to recommend in this situation. I believe my trainee used the right traffic call using 'one thousand feet below you' instead of 'traffic at FL350.' I was told that the charts show the STAR arrival as a profile descent; but I personally haven't seen the charts. Maybe the charts need to be updated if that is the case.
Enroute Controller described a loss of separation event that may have involved the issuance of altitude phraseology and misunderstanding of same.