This rule is part of the Axis & Allies Global 1940 2E, House Rules Expansion.
Naval Task Forces at sea brought with them Support & Supply Ships - in order to perform their operations - and make necessary repairs without depending on a Port.
This is reflected in the Capital Warship Damage rule.
The largest Battleships ever built, were limited in numbers - but extremely powerfull.
This is reflected in the Super Heavy Battleship rule.