Looking for an example of "plain text" which is obvious to anybody, it seems to me that the "Subject" field of e-mails is a good example. Common e-mail software lets you enter any text but gives you never access to any higher-level protocol. Possibly you can select the font in which the subject line is shown, but this is completely independent of the font your subject line is shown at the recipient. Thus, you transfer here plain text, and you can use exactly the characters which either Unicode provides to you, or which are PUA characters which you have agreed upon with the recipient before.
In fact, the de-facto-standard regulating the e-mail content (RFC 2822, dated April 2001 http://www.ietf.org/rfc/rfc2822.txt , afaik) defines the content of the "Subject" line as "unstructured" (p.25), which means that is has to consist of US-ASCII characters, which in turn can denote other (e.g. Unicode) characters by the application of MIME protocols. Thus, the result is an unstructured character sequence. There is e.g. no possibility to include superscripted/subscripted characters in a "Subject" of an e-mail, unless these characters are in fact included as superscript/subscript characters in Unicode directly. Thus, proving the necessity to include a character in the text of a "Subject" line of an e-mail, is proving that the character has to be available as a plain text character. If, additionally, the character is used outside a closed group (which can be advised to use PUA characters), then there is a valid argument to include such a character in Unicode. Is my assumption correct? (I think of the SUBSCRIPT SOLIDUS proposed in WG2 N3980. It is in fact annoying that you cannot address DIN EN 13501 requirements in an e-mail subject line written correctly, as Unicode, although being an industry standard, until now did not listen to an industry request at this special topic.) - Karl
