The main mysteries include who is the lucky naval customer and how is the HDS-400 propelled?
- Taiwanese Navy. This might explain South Korea's secrecy. It would be easier to build and hand-over a submarine to Taiwan before China decides to publicly object. Just 400 tons makes an HDS-400 a defensive submarine.
- the US Navy may want to trial the intelligence gathering and special forces value of a mini HDS-400 submarine. Also the US could use it in ASW exercises - given opponents North Korea and Iran widely use mini-submarines.