Increments per exchange, and a reply must carry the counter of the message it answers.
The anti-replay mechanism for the handshake itself. Messages 1 and 2 share one value and messages 3 and 4 share the next, which is how you pair them up in a capture containing several handshakes at once.
Key Replay Counter
Authored in EAPOL-Key descriptor . Drawn in 4 frames.