Archive for the Sony MBX-182 mainboard, with M721 given as the project code. Sony uses MBX numbers for laptop boards built by external ODMs, and an M-prefixed code like this one is likely the maker's project name. The 1.21 MB RAR probably holds a schematic used to trace power rails, locate test points and diagnose shorts or missing voltages. No revision is stated, so check the silkscreen on the PCB, and open the file with WinRAR or 7-Zip before comparing component references to the 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 @generalschematics. 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.