This is a lists of all the constraints that must be supported: Each Observation must support: a result value -> if the result value is a numeric quantity, a standard UCUM unit a reason if the value is absent* component results There is no "OR"  operator implied between the list of items. You have to support these 3 things. Either individually or in concert with others. (There are detailed additional  constraints defined by the formal definition and in the profile-specific guidance (where the asterisk prompts the reader) Agree that the wording could be clarified and will add more explanation about the profile introductions in the "how to read this guide" section here: http://hl7.org/fhir/us/core/index.html#how-to-read-this-guide