There are 2 file types use the .JST file extension:
1. Jnes Save State
Save file created and used by Jnes, a Nintendo Entertainment System (NES) emulator for Windows; contains a save state of an NES game.
The JST file is the default save state used by Jnes. The emulator also uses the ".js0" to ".js9" extensions for the other 10 save slots that it supports. All save states are located in the \saves subdirectory of Jnes. If you move them from this location Jnes will not read them.
Updated: January 15, 2017
2. JavaServer Page Document
Server-generated Web page written in the JavaServer Page (JSP) language; commonly contains HTML code in addition to the JSP code; compiled into a servlet that is executed by the Java-enable Web server and then delivered to the user as a Web page.
JavaServer Page documents much more commonly use the .JSP extension.
Updated: August 21, 2010