Reference PDF for the 2SD973, 53.5 KB, with an _E suffix the name leaves undefined. It sits in the schematic category, but the JIS 2SD prefix marks an NPN transistor for low-frequency service, so it functions as a datasheet. Technicians use it to identify the emitter, base and collector on the package, review breakdown voltages and current limits, and check gain ranks. It is a quick sanity check before fitting a cross-referenced part found in a substitution guide.
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.