Compressed archive labelled only with Apple board number 820-1398, with no model, revision or contents listed in the name. Expect repair documentation such as a schematic or a board layout, and check what is inside after unpacking with WinRAR or 7-Zip. Apple stamps the 820 logic board number on the PCB itself, which is the most reliable way to confirm a match. Once confirmed, the material helps trace voltage rails, locate components and narrow down shorts on the motherboard.
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.