My guess, for what it is worth, is that it is related to what a controller will say when passing radar vectors to you. Telling you to "come right, track 020" is fairly useless as neither of you know what the wind is, so they'll tell you "come right, heading 020" - if the track is important, as in a SRA, they can pass further corrections. That may be rubbish though
But in the real world i'd ignore what CAP413 says about passing heading information. As you'll often see ATCOs say here, they would like to know "who you are. where you are. what you want". Heading information is not really important, and I've never heard a CAP 413 response to "pass your message" (even Genghis, who has the most by-the-book RT that i've ever heard, doesn't do it
). If they need to know they will ask.
HTH