The 2SD2129 transistor is documented in this 216 KB PDF datasheet, catalogued among schematics. Following JIS numbering, 2SD parts are NPN transistors for low-frequency applications such as power switching, regulators and amplifiers. A technician refers to it for ratings, characteristic curves, gain rank letters and pinout before testing or replacing the part. Confirm the package style on the board matches the outline shown, because the same type can appear in more than one case.
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.