Club boats should be used when there are boats that can be shared and sailed by multiple helms, for example your club has a fleet of boats for hire. When calculating series results, if a helm has sailed multiple clubs boats of the same class, they will be treated as the same boat.
As a quick example, if Bob sails:
⛵Topper "club 01" in Race 1
⛵Topper "club 06" in Race 2
⛵Topper "club 02" in Race 3
⛵Topper "club 01" in Race 4
Then all his results count to the series results as through they were a single Topper.