This case-insensitive code system https://www.ehealth.fgov.be/standards/fhir/referral/CodeSystem/be-cs-co-prescriber-choice defines the following codes:

Code	Display
mandatory	The co-prescriber must sign
one-of-mandatory	The signature of the co-prescriber counts in the number of required signatures, but not blocking
recommended	The signature of the co-prescriber is recommended, but not blocking