Reefing is the way to reduce sail area quickly when your boat becomes overpowered. When buying a mainsail, your UK Sailmaker will determine how many rows of reefs are needed. Buying more reefs than necessary adds expense and extra weight to the sail.
Most buoy racers buy a sail with a single reef or no reefs at all, because they are concerned about adding too much weight to the sail. A single reef will be used on most boats in winds around 25 knots since most racers put up smaller genoas before reefing.
A second reef is used in winds blowing close 30 knots — conditions where most day races have been cancelled. Those doing distance races need a second reef.
A third reef is only used by sailors who venture offshore.