Suunto DX has one mode dedicated to rebreather diving, CCR mode. This mode uses fixed high/low setpoint values which you can modify in the watch or through DM5.
Fixed setpoint calculation enables Suunto DX to be used as a backup dive computer on rebreather dives. It does not control or monitor the rebreather unit in any way.
When you select CCR mode (see CCR mode), the gas settings split into two: CC gases (closed-circuit gases) and OC gases (open-circuit gases).
For rebreather dives, Suunto DX should be used as a backup device only. The primary control and monitoring of your gases should be done through the rebreather itself.