Order of Exchanging Revocation Keys during Lightning Transactions
Alice and Bob jointly create a public key R for each revocable output (RSMC output script).

They each privately generate r-1 and r-2, and derive public keys R-1 and R-2 from these. R is the sum of R-1 and R-2. No secrets have been exchanges, only public keys. So R is unspendable by either. However, it is spendable by r = r-1 + r-2

When a new commitment tx is created, this RSMC must be revoked. If the new tx commitment favors Bob, then Alice must share r-1 with Bob. Now bob knows that he can spend this revoked output if Alice broadcasts it. Bob doesnt need to share r-2 with Alice as he would rather broadcast the subsequent tx favoring him.

Pre-images are used for routing (htlc output scripts) See later slides in chapter: https://teachbitcoin.github.io/payment_channels.html#/