The Results profile currently states that the category must contain code 'laboratory' with system 'http://hl7.org/fhir/observation-category'. It's enforced using a FHIRPath/XPath invariant.

This could be enforced using slicing - which would eliminate the need for an invariant. In fact, this is how the VitalSigns profile does it. See: http://build.fhir.org/vitalsigns.html

For consistency with VitalSigns (which is adopted by US Core), the results profile should use slicing to constrain the category. Otherwise implementors need to support both ways to do what is essentially the same thing.