NJS, or Nihon Jitensha Shinkōkai, was the Japanese Kerin race sanctioning body. Whilst its functions have now been taken over by JKA Foundation the accreditation process remains to ensure compatibility of equipment in Kerin races. The remit of the NJS to ensure products are compliant with track racing meant that equipment is typically of very high quality and interoperable amongst other NJS stuff, however there are a few quirks such as the headset size and seat rail spacing that make it problematic to use certain NJS items with the bike you might already have.
The other consideration with NJS items concern the bottom bracket and headset bearings which will tend to be caged design rather than cartridge. This makes regular servicing a must with the British climate.