Archive for the Apple logic board 820-00840, revision A as given in the name. The name does not state whether it contains a schematic, boardview or firmware, so open the RAR and check the contents first. Apple prints the 820-xxxxx number on every logic board, which is the main way to identify the exact board in a MacBook regardless of model name. Files of this kind help trace power rails, locate components and test points, or restore a BIOS chip, but the PCB number and revision must match before relying on them.
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 @schematicslaptop. 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.