This type of flange consists of both a stub end and a flange.The flange itself is not welded but rather the stub end is inserted / slips over the flange and is welded to pipe. This arrangement helps in flange alignment in conditions where non-alignment may be an issue. In a lap joint flange, the flange itself is not in contact with the fluid. The stub end is the piece which gets welded to the pipe and is in contact with the fluid. Stub ends comes in type A and type B. Type A stub ends are most common. Lap joint flange only comes in flat face. People confuse lap joint flange with slip on flange as they look very similar with the exception that lap joint flange has round eges on the back side and a flat face.