An imapflags question

Several places in the imapflags draft, there are flags such as this one,
starting with a $ rather than \\

           addflag "MyFlags" ["\\Answered", "$MDNSent"];

I presume that this is not a variable, but actually a flag that begins
with $. Where in the specs did those come from? Is this the "keywords"
type flag that RFC 3501 allows?

If it is the "keywords" type flags, I think it would it be prudent to
make reference to that aspect of RFC 3501.

Thanks!
Aaron