Small boardview archive for a Quanta board with the project code BK5, with no brand or full board number in the name. Quanta uses short project codes like this, and the full board number usually takes the form DABK5xMBxxx. A boardview maps component placement and nets, helping a technician find a shorted part, a missing rail or a test point quickly. Extract it and open the file in a viewer such as OpenBoardView. Check that the board in hand carries the BK5 code before relying on it.
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 @BoardSchematic. 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.