What is the difference between Exchange & Offer in the Monetary System for NXT / Ardor Platform?
Please refer to the official monetary system documentation https://bitbucket.org/JeanLucPicard/nxt/issues/205/monetary-system-documenation section "Currency Exchange".
The NXT wiki also has an excellent walk-through of the process in https://nxtwiki.org/wiki/Monetary_System#Exchange_an_Exchangeable_Currency