The 2SJ339 appears in this small PDF of roughly 40.7 KB. The name carries only the part number, with no hint of the equipment it came from. Japanese JIS coding uses 2SJ for P-channel field-effect transistors, so the content is most likely a condensed datasheet. Technicians rely on such sheets for gate threshold, drain current limit and pin order when diagnosing a power path or a replacement. Confirm the component marking before substituting another type, since different P-channel parts vary in ratings.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @cyberlanelectonik. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.