The AMR (AMap Rebuild) Info dialog interprets and displays an undocumented structure specific to the AMap Rebuild process. This structure is located at 0x4400 in the file (the same location as the first AMap page in the file, which by
definition is invalid if this structure exists) and is where information about the build (mainly, it's progress through the file) is stored. It's purpose is to allow rebuilds to "resume" across multiple sessions of Outlook.
This is an undocumented structure, which can (and does) change between versions of Outlook. This tool was augmented with the ability to read this structure as the feature was being developed.
The file range control at the top of the file indicates the progress the build has made through the file. Rebuilds start at the end, and work their way to the start. In the picture shown, a rebuild is not in progress.
The table view below the file range control lists the fields of this structure and their value, if present.