UMLNIEM3 — Clause 8 explanation needed

The descriptions "Rule is definitional" and "Rule is not reliably computational" occur several times starting on page 153. What exactly do they mean? Also, I assume this is not a description of the element since they are all unique. Therefore is it an editorial comment on the rule itself? Or an explanation as to why there is no OCL? If either one, a general terms and definitions section at the beginning of the chapter would be useful. Also, Constraint is non-computable and The constraint is realized through provisioning. And “Constraint expression as OCL is deferred”: until when is it deferred?
For this and other reasons, Chapter 8 really needs an introduction to explain what it is, how it works, conventions followed and so forth. As it stands, it is expert friendly: you understand what it means if you understand what it means.

Clarify clause 8

Introduce an introductory paragraph into clause 8.1 to assist the reader in understanding the remainder of clause 8.