Compact 247 KB RAR archive referenced only by the Apple board number 820-2055, with no schematic number, revision or board type in the name. Its small size suggests a minor daughterboard rather than a full logic board, though the contents are unconfirmed until opened. Apple uses 820-xxxx for PCB part numbers, and matching that code to the silkscreen on the board is the first check. Schematic or layout data of this kind helps trace connector pinouts, rails and signals between the small board and the main board.
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.