What do the Umpire's white flag signals and the "Stop" command mean for a crew?

From the By-Laws to Rule 80:

• White flag shown, crew named, flag lowered to one side: the crew is about to interfere with another by its wash or by leaving its lane, and must change direction towards the side the flag is lowered. The Umpire does not otherwise give steering indications unless there is an obstruction in the lane.
• White flag raised vertically, crew named, "Stop": the crew must immediately stop its boat. This is used to ensure safety and prevent damage to boats and equipment. The crew may begin rowing again to finish the race if the Umpire permits.

At the finish (Rule 82 By-Laws):

• White flag to the crews: the Umpire considers the race was in order. Crews who disagree raise an arm to object before this.
• Red flag: the Umpire has decided the race was not in order, or is dealing with an objection, and will announce the decision.

At the start (Rule 77 By-Law): a bell and a waving red flag (or flashing red light and repeated audible signal) stop the race after a false start.

An excluded crew is told "(name of crew) - (reason for the exclusion) - red Card - Exclusion!" The Umpire must also make sure a crew they want to address can hear them.