Core: Additional "extension" logic

[jerstlouis]

• We need to clarify that one use of overriding may be to define a more basic symbology behavior, then override with a rule (which might using unsupported constructs) that uses a more complex requirement class
• We need a system identifier for capabilities for use in selectors to decide which rule to apply based on:
  • The supported requirement classes
  • Display characteristic such as 3D, VR (stereoscopic), AR (seeing the real world though) e.g., @chris-little 's example of the different symbologization for bridges in 3D vs. 2D -- should these be requirement classes in their own right in a future part?