In the table of elements defining a property, it states:

The type of the property value. Properties of type "code" contain a code defined by the code system (e.g. a reference to another defined concept)

It is unclear which code system is being referenced here: The "current" CodeSystem resource in which this property is defined; the code system which property.code is drawn from, or another code system containing concepts for the property values.

I think it is the latter option, but it isn't obvious. (It's also strange that the codes from that code system are used, but the system itself is never referenced.)