These cartridges when transported separately are covered by ADR and consequently excluded from PED due to Article 1 paragraph 2(s).
If such cartridge is used in a fire extinguisher (a PED assembly) it is in the scope of the PED directive .
For further specific information see PED Guideline C-20.
Note:
see PED Guidelines A-01 and B-14
*) The term used in the context of the ADR is different depending on size: propellant gas cartridges can either be (non-refillable or refillable) cylinders or (non-refillable) gas cartridges.