Sealed & Abstract Class in Delphi - ablealias/Delphi GitHub Wiki

If a class is marked sealed, then it cannot be extended through inheritance. If a class is marked abstract, then it cannot be instantiated directly using the Create constructor. An entire class can be declared as abstract even if it does not contain any abstract virtual methods. A class cannot be both abstract and sealed.

type
className = class [abstract | sealed] (ancestorClass)
  memberList
end;