It means that that copy of the book has already been assigned to a different Adobe ID.
Each copy of a book can only be used on one Adobe ID.
The .acsm file is just a token for allowing the real book to be accessed.
Once the .acsm file is used, a DRM locked .epub or .pdf file is downloaded, locked to the Adobe ID in use at the time.
After that, none of the files can be used on a different Adobe ID.
Interesting. Typically when that occurs though, we'll see an error message stating: E-LIC_ALREADY_FULFILLED_BY_ANOTHER_USER