The 3N253 is documented in this 42.2 KB PDF, categorised as a schematic but evidently a discrete-component datasheet. Its JEDEC 3N prefix identifies a registered four-lead semiconductor, and identical sizes up to 3N259 imply a combined family document. A technician would turn to it for lead identification, electrical limits and outline drawings when testing or sourcing a replacement. Open it with a standard PDF reader, and ensure the grade printed on the installed part matches the one you are reading.
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.