Comment:

"Note that HL7 does not generally define "modifier" extensions - if HL7 publishes an element that modifies the meaning of other elements, it will generally be part of the resource content itself, since everyone has to understand the extension anyway."

Remove this wording as it's not true. We've defined 5 modifier extensions so far where we've decided that most systems don't support something, but a few do. And we'll undoubtedly define more, especially in implementation guides.

Summary:

HL7 does define modifier extensions