I believe that there is a statement that any productions that
cannot be found are defined either in the base ABNF spec or in
5322.
SM is (as usual) correct. Section 4.1.2 of 5321 says:
Terminals not defined in this document, such as ALPHA, DIGIT, SP,
CR, LF, CRLF, are as defined in the "core" syntax in Section 6 of
RFC 5234 [7] or in the message format syntax in RFC 5322 [4].
RFC 5322 has the definition of atext, so there's no dangling
reference. This is at most an editorial issue, where it might have
been a good idea to be more specific about what symbols are imported
from where, so those of us doing a grep for "atext" would find it.
R's,
John