Component documentation for the 4N38 optocoupler, filed under schematics though it is in practice a part datasheet in PDF form of about 140 KB. The 4N series are LED-input, phototransistor-output isolators found in power supply feedback loops, interface boards and industrial controls. A technician uses such a sheet to check pinout, LED forward current, isolation rating and current transfer ratio before swapping a suspect isolator. Compare the marking and package on the board with the drawing before substituting a part.
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.