Constant REQUEST_CANCELLED 

pub const REQUEST_CANCELLED: u64 = _; // 1u64

The application error code a request stream is reset with when the requester abandons it: CANCELLED, 0x1.

Draft-17 removed UNSUBSCRIBE and FETCH_CANCEL. Cancelling a request is resetting the bidirectional stream it was made on, and CANCELLED is the code draft-17 assigns for “the subscriber or publisher cancelled the Request” — see DataStreamResetErrorCode::Cancelled. Taken from the codec’s own registry rather than written as a literal so a renumbering in a later draft cannot be missed here.

This is what RequestStream::cancel uses when no code is chosen for it, and what RequestStream’s Drop sends.