Logic board schematic for Apple project X362, board number 820-00239, in a 5.31 MB RAR. Apple marks its logic boards with 820 numbers, and the matching number silkscreened on your board must be confirmed before use. The schematic shows power rails, enable signals and connections between ICs, letting a technician follow the power-up sequence, find what pulls a rail down and identify component values. It is normally a PDF and pairs well with a boardview for locating parts physically.
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 @hrtechno. 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.