It means a groove has been machined around each combustion chamber in order to use a special head gasket that has a wire sealing ring. They are typically for boosted/nitrous engines, although it would seem that with good fasteners and a good tune they arent that necessary. Many think of blowing regular gaskets as a safety feature since the gasket pops instead of a piston or rod.
Its means there has been an O ring groove machined into the surface of the head, to accept O ring gaskets, which have an O ring that gets pressed into the O ring of the head to form a tighter seal between the block and the head. Normally used with nitrious and boost applications.
although it would seem that with good fasteners and a good tune they arent that necessary. Many think of blowing regular gaskets as a safety feature since the gasket pops instead of a piston or rod.