Sony notebook schematic package for the MBX-155 motherboard, with MSS1 named as the project code. Sony uses the MBX prefix for its laptop boards, which are built by external manufacturers. The 1.29 MB RAR is a typical size for a compressed PDF diagram used to trace supply rails, find test points and check for shorted capacitors around the CPU and chipset. No revision is stated in the name, so confirm against the silkscreen on the PCB, and consider the related revision 0.30 file for comparison.
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.