576k is reserved for program space - the built in applications execute in place from ROM. However, some RAM is needed for instance data and heap space for each app, and some RAM is used by any add-on applications that don't XIP. Up to nine applications can be cached in the 576k program RAM at any one time.
32k is used for video RAM
406k is set aside for ramdisk. By default, some sample apps and blank documents are created when you first boot the unit or launch the apps. You can recover some ramdisk space by deleting the samples and templates. State files also take up ramdisk space, and you can recover some of that space by warm booting the unit.
Steve