Apple logic board 820-3787 revision A, supplied as a 3.8 MB RAR archive with no further detail in the name. Expect schematic and possibly boardview material once unpacked; boardview files open in tools such as OpenBoardView or FlexBV. Apple prints the 820- number on the PCB, so match it and the revision letter before working. Typical uses include measuring rails against reference values, locating test points and isolating shorted capacitors around power circuits.
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 @Mac_Shematic_Santale. 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.