Are the value sets defined by HL7 or it is the implementers who choose to define their own values? If HL7 sets defines value sets, are the implementers supposed to use only all or some of the value sets there, or they can add and modify the value sets as they see fit?
Value sets can be defined by HL7, other SDO or locally, although, if you want your values to be understood outside your organization, SDO defined value sets are recommended.
HL7 defines values sets, and if you use them, only already defined values can be used.
Hope useful, Martí