NXP document with the identifier UM10391, provided as a PDF of about 1 MB and indexed here under schematics. The UM prefix is the code NXP uses for its user manuals, which normally cover a microcontroller family's registers, peripherals and pin functions. The name does not state which chip it covers, so open it and confirm that the part number matches the IC on your board. Technicians use such a reference to check pin assignments, supply and clock requirements, and programming or debug interfaces when diagnosing a board built around that part.
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 .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.