The 3N258 is the subject of this 42.2 KB PDF, indexed as a schematic but in practice a component reference. Its 3N prefix places it in the JEDEC registration for semiconductors with four leads. Nearby numbers in the 3N25x range share the same file size, which points to one datasheet spanning several related grades. A technician reads it for pin order, voltage and current limits and package outline when testing or replacing the part. Open with any PDF reader and compare against the marking on the board.
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.