Modifier elements are elements that change the meaning of the resource as a whole, or change the meaning of other elements in the resource.  E.g. ServiceRequest.doNotPerform inverts the meaning of the ServiceRequest.  Patient.deceased=true means that the patient can't really receive care anymore. "shallConformWith" doesn't change the meaning of the request it's on, nor does it change or invalidate any of the other data elements on the ServiceRequest.  There's no question it's an important element, and if a system were to ignore it, they'd definitely want to make sure the narrative for the instance was available to the reader.  However, we don't flag things as modifiers just because they're important.  (E.g. MedicationRequest.medication isn't a modifier, nor is ServiceRequest.code - even though both are obviously vitally important to understanding their respective resources.)