Former avis employee here, the only code that we trained counter staff to be on the lookout for across the entire company were the State of FL codes. Most other codes will probably be able to slip by on a preferred reservation, but there are agents throughout the operation that will spot obvious misuses of corporate codes. In the NY Metro area the biggest culprit during my time were folks using the USAA codes without any affiliation to USAA to waive underage fees