Short 347 KB PDF about the MMC memory card standard; the bracketed number in the name suggests a duplicate saved copy, and no maker or version is stated. Expect information on the card connector pinout, SPI mode communication, commands and responses. Such a reference is handy when tracing a card socket on a board, checking signal levels or debugging firmware that fails to initialise a card. Compare its contents with other MMC documents in the index, as it may be a different edition of the same material.
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 @datasheet. 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.