tags: add text to distinguish IANA assigned tag names and all the others

Author: robUx4

cellar-tags/tagging.md

@@ -74,6 +74,8 @@ which tag has the desired meaning so that other apps could understand the same m
 
 ### TagName Formatting
 
+The `TagName` element can hold any UTF-8 data. However, to distinguish between the IANA assigned names and custom or private ones, a set of rules are defined.
+
 Assigned `TagName` values **MUST** consist of latin capital letters, numbers and the underscore character '_'.
 
 Assigned `TagName` values **MUST NOT** contain any space.