The CarePlan resource currently states that the category must contain code 'assess-plan' with system 'http://hl7.org/fhir/us/core/CodeSystem/careplan-category'. It's enforced using a FHIRPath/XPath invariant.

This could be enforced using slicing - which would eliminate the need for an invariant. Is there a best practice that would advise whether to prefer slicing or an invariant for this? I always assumed that if it could be expressed without an invariant, then it should be expressed without an invariant – but perhaps that is just a personal preference.