Comment:

"it means that some developer has provided appropriate instructions for what to do with the data contained in it because of the existence of the modifier extension" -> "it means that some developer has provided appropriate instructions for what to do with the data contained in it because of the existence of the modifier extension or has determined that the modifier does not impact on the system's computational functions. Note that this assessment must be repeated each time a system's computational behavior changes."

Existing wording implies there must be special logic, and there won't always be. Also, we need to caution about the need to think about modifier extensions every time.

Summary:

Revise text on "understands this extension"