question on RDFa subject resolution

The following seems oversimplified, but it does seem to reflect the
behavior I see in the RDFlib implementation of RDFa: when an RDFa
processor is looking for a subject to go with a predicate and an object,
it searches up the ancestor axis until it finds either an about attribute
or an id attribute. The first of either that it finds becomes the subject
for the triple.

If this is an oversimplification, what am I missing? The syntax document
is a bit vague on the potential role of id attributes as subjects as
compared with about attributes.

thanks,

Bob