Section 14.1 states:

      It is a processing error if the SequenceContent region of an empty sequence has non-zero length when parsing.

How is it possible for an empty sequence to have a non-zero length of its content region?

I understand it could have delimiters outside the content region, and other framing possibly, but the content region has nothing in it then only annotations can appear.

Annotations can be property definitions or can be DFDL statements. None of those can consume representation bits when parsing, or produce representation when unparsing.

So I believe this processing error cannot happen.

If I am correct, the statement about processing error should be dropped.

Also, clearly this statement does not apply to a sequence that carries a dfdl:hiddenGroupRef, since the hidden group can contribute to the content region. I would not consider a sequence carrying hiddenGroupRef to be empty unless the referenced group is also empty.