By computing standards, this is a very old file format used to record shogi games. Shogi is Japanese Chess. It is a challenging cousin to chess. Players of chess often see large improvements to their chess skills by studying shogi.
This project is hosted by SourceForge.net. The project team describes it as:
A free Java based reference library for reading and saving Portable Shogi Notation (psn) files. Also, a document explaining the creation of '*.psn' files and their relation to '*.kif' files.
The official white sheet for *.psn file format is found at the japanesechess.org site.