Major Section 4.2.1 Model Diagram reads in part: ***** The class ยท ToleranceValueType is comprised of a set of optional attributes of DurationType. Tolerances can be expressed in any combination; however, the complexity of rules addressing the relationships of tolerances in start, end, and duration will likely lead to implementation-specific rules limiting the concurrent uses of tolerance attributes. ***** It is difficult to tell is this is expressing a normative requirement or is it simply commentary along the way? The majority of section 4 reads this way so take this as a single example.