Apple iPhone SE document in a 919 KB PDF whose Russian name means resistance measurement, so it most likely lists diode-mode or resistance readings for lines on the logic board. Technicians compare such reference values with their own measurements to locate shorts or open circuits without powering the phone. It opens in any PDF reader. Readings depend on meter type and mode, so treat values as guidance and confirm the board generation matches the one under repair.
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 @mobileschematicshub. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.