[ENH] clarify guiding principles for requirement levels (#1444)

* [ENH] clarify guiding principles for requirement levels

* remove backticks, add metadata

Co-authored-by: Chris Markiewicz <[email protected]>

---------

Co-authored-by: Chris Markiewicz <[email protected]>

Author: sappelhoff

src/common-principles.md

@@ -6,8 +6,17 @@ The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
 "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
 interpreted as described in [[RFC2119](https://www.ietf.org/rfc/rfc2119.txt)].
 
-Throughout this specification we use a list of terms and abbreviations. To avoid
-misunderstanding we clarify them here.
+Based on these keywords, we define three requirement levels for specifying data or metadata:
+REQUIRED, RECOMMENDED, and OPTIONAL.
+The guiding principles for when particular data is placed under a given requirement level
+can be loosely described as below:
+
+* REQUIRED: Data cannot be be interpreted without this information (or the ambiguity is unacceptably high)
+* RECOMMENDED: Interpretation/utility would be dramatically improved with this information
+* OPTIONAL: Users and/or tools might find it useful to have this information
+
+Throughout this specification we use a list of terms and abbreviations.
+To avoid misunderstanding we clarify them here.
 
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