RAR archive of 6.03 MB for Apple logic board 820-01598. The file name states only the board number, so the exact contents are not specified, though the size suggests a schematic PDF, possibly bundled with a boardview. Apple prints its 820-xxxxx identifier on the main board, and that number plus any revision suffix should match the unit on the bench. Schematics help with tracing power rails and enable signals, while boardview files open in OpenBoardView or FlexBV to locate components and test points.
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 .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.