the "a" in "RDFa"?

I believe I've asked this before, but I'm pretty sure I didn't see an
answer, and web searching can't find my original question.

Where did the name RDFa (or before it, RDF/a) come from? Take "RDF" and
add an "a"--why? Does it mean "assertion"? Or does it represent the HTML a
element, which is a popular element in RDFa syntax for wrapping PCDATA to
make oy the object of an RDF triple? Or does it stand for a slightly
altered fork of RDF the way you might add step 2a between step 2 and step
3?

thanks,

Bob