This document discusses the legal doctrine of estoppel. Estoppel is a principle of fairness that prevents a party from reverting to strict legal rights if doing so would cause hardship to the other party who reasonably relied on promises or assurances made by the first party. The document outlines the three conditions typically required for estoppel to apply - a representation was made through words or conduct, it was intended to affect the legal relationship, and the other party relied on it to their detriment. It provides examples from arbitration cases where estoppel prevented an employer from recovering overpayments or making changes to policies due to longstanding practices the union relied on.