Apple logic board schematic for board number 820-3023, provided as a PDF inside a 1.42 MB RAR. Apple identifies logic boards with 820-xxxx numbers printed on the PCB, and the specific Mac model is not stated in the name, so match the code on your board. Technicians use these diagrams to follow power rails, check the power sequencing and locate the source of a short. Any PDF reader will open it, and pairing it with a boardview for the same number makes finding parts faster.
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 @LaptopsBoardview. 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.