Constant STREAM_ABANDONED 

const STREAM_ABANDONED: u64 = _; // 0u64

The application error code a namespace subscription’s stream is abandoned with when this endpoint never served it: INTERNAL_ERROR, 0x0.

Draft-16 assigns no code for this. Its only registry of stream error codes is Section 13.4.4, “Data Stream Reset Error Codes”, and every entry there is specified by Section 10.4.3, which is about closing subgroup streams — a namespace subscription’s stream is not a data stream. Section 6.1 offers a FIN as the alternative and names no number for the other form.

So this is a choice rather than a citation, and it is the one that claims least: INTERNAL_ERROR is “an implementation specific error”, which is exactly what a stream abandoned mid-accept is. It is taken from the codec’s own registry rather than written as a literal so a renumbering in a later draft cannot be missed here.

Only the paths that give up on a stream before it carries a subscription use it — a request that could not be built, a first message that could not be read, a Request ID the peer may not use. A caller cancelling a live subscription picks its own code, or uses the FIN form and picks none.