The flat face flange has a gasket surface in the same plane as the bolting circle face.
This type of flange is widely used in KS and JIS standards, and therefore, can be easily procured in the Korean market.

Additional Information
Compared with FF and RF, RTJ, T&G and M&F provide stronger connection and better fit. However, they are more expensive than FF and RF, and commercial availabilty can be limited since FF and RF are most commonly used in the market.
In addition, when using T&G and/or M&F, the complexity of designing the piping system may increase since they have to be matched.
Same surface type should be used and it is not recommended to assemble with a different surface type (e.g., assembling RTJ with RF).
When not using the same surface type, a gap may exist in the gasket part and a trustable connection cannot be guaranteed, and thereby a leakage may occur.