All #IMPLIED means is what it says here, that this attribute, while it's allowed for this element type, doesn't always have to be there, and that if it isn't, there's no default value, it just isn't there.
The term #IMPLIED is inherited from SGML, which explains that in this case the application should "imply" the attribute's existence or nonexistence; the discussion is somewhat metaphysical in tone and has puzzled generations of students of SGML.
The real reason #IMPLIED exists, of course, is to make declarations easy to parse; every attribute declaration has a name part, a type part, and a default part (which may, if it's #IMPLIED, be effectively absent).
Copyright © 1998, Tim Bray. All rights reserved.