Which point is included in al-Nawawī’s explanation of ʿillah?

The Muqaddimah’s Aim: Mastery, Selection, and Critical Comparison

• Every visible wording difference between apparently sound routes
• Only a break in transmission expressly apparent in the chain
• A consequential defect in chain or text uncovered despite apparent soundness
• Every unique addition by a reliable transmitter, even when corroborated