Be more explicit about "elective" and "critical"

draft-ietf-cbor-time-tag.md

@@ -273,7 +273,9 @@ item (CBOR major type 5).  The map may contain integer (major types 0
 and 1) or text string (major type 3) keys, with the value type
 determined by each specific key.
 For negative integer keys and text string values of the key,
-implementations MUST ignore key/value pairs they do not understand.
+implementations MUST ignore key/value pairs they do not understand; we
+say that these keys are "elective", as the extended time is still
+usable if an implementation elects not to implement them.
 Conversely, for unsigned integer keys, implementations MUST signal as
 an error key/value pairs they do not understand or implement
 (these are either "base time" or "critical", see below).
@@ -284,11 +286,12 @@ text-string keys, which may encode supplementary information.
 
 Supplementary information may also be provided by additional unsigned
 integer keys that are explicitly defined to provide supplementary
-information ("critical"; as these are required to be understood, there
-can be no confusion with base time keys).
+information (we say these keys are defined to be "critical"); as these
+are required to be understood, there can be no confusion with base
+time keys.
 
 Negative integer and text string keys always supply supplementary
-information ("elective", and this will not be explicitly stated
+information (they are "elective", and this will not be explicitly stated
 below).
 
 Supplementary information may include: