Define rule DDF99975: An Activity should refer to at least 1 Procedure, BiomedicalConceptSurrogate, BiomedicalConcept or BiomedicalConceptCategory

CHK0057: An activity should refer to at least 1 procedure, BC surrogate, BC or BC category
An activity can be of one of these types

- Procedure (0..*)
- BiomedicalConcept (0..*)
- BiomedicalConceptCategory (0..*)
- BiomedicalConceptSurrogate (0..*)

This means that an activity must reference at least one of these classes. I'm not sure whether an activity can reference a mixture of different types.

Start by locating the DDF99975 or CHK0057 definition and the activity model or validation entry point. Clarify whether one activity may mix reference types, then enforce the requirement that at least one listed reference exists; done means the rule is defined and its behavior is covered by the project's validation checks.

Written by the indexing model from the issue text.