In FHIR, the StructureDefinition.context specifies where an extension can be attached. Many extensions can be attached to any "Element".

For example, see CQFM Artifact Comment with an example

The "context" for that extension is

"context": [
    {
      "type": "element",
      "expression": "Element"
    }
  ]

which means it can be attached to things that of type "element" and the expression says where it can be attached, in this case any "Element".

It would be nice to have a generic user interface component that provides a way to enter the "text" (valueMarkdown) and radio button for "type". The target, reference and user component extension are probably not necessary to support.

Then enable this component to be used on any or selected "elements" in the Measure resource (metadata). For example, Measure.improvementNotation is an Element, and the CQFM Artifact Comment extension can be used to explain more about the improvement notation, especially if the current code choices (increase, decrease) are not directly applicable to a measure (such as CMS334).