Complex sentence: It is noteworthy that a small triangle inside the northern side of the agreed joint development area was also subjected to an overlapping claim by Vietnam and in 1999, Malaysia, Thailand and Vietnam agreed to apply the joint development principles for this area.
Split into simpler sentences: It is noteworthy that a small triangle inside the northern portion of the joint development area is also subjected to an overlapping claim by Vietnam. In 1999, Malaysia, Thailand and Vietnam agreed to apply the joint development principles for this area.