The FHIR specification supports different operations that require evaluating expressions with FHIRpath and/or CQL expressions. These expressions require access to the context of the operation meaning the parameters provided.

I suggest to add a generic section on Expression generation and state in there that all parameter fields will be accessible as enviroment variables.

%param --> Parameter resource

In the case of ActivityDefinition.$apply, this will result in the following environment variables: %activityDefinition, %subject, %encounter, %practitioner, %organization, %userType, %userLanguage, % userTaskContext, %setting, %settingContext.