@Zharif
It is a mess.
The explanation by Olof about IMDISK was that it connected over (or below, cannot remember) Mount Manager (as opposed to good ol' VDK, the KernSafe Total Mounter and - later - the same Olof's Arsenal thingy)
Unfortunately when we are talking of devices, volumes, partitions, filesystems, "disk extents" (what I call what erwan.l calls "slices") are not entirely "exact" terms as they belong to a gray area and the exact shade of gray depends oin how you look at them.
A couple of links (one serious, one not so much ) for your interest:
http://reboot.pro/to...ated-with-dsfo/
http://reboot.pro/to...lack-and-white/
@erwan.l
As always I may be wrong, but let's go into practice (besides theory).
Let us go back to the assign verb, possible cases:
1) user is Admin and wants to mount a volume[1] to a drive letter -> OK, granted
2) user is Admin and wants to mount a device to a drive letter -> OK, granted
3) user is Admin and wants to mount a device that does not ALSO expose a volume (such as an IMDISK device) to a drive letter -> OK, granted, see #2 above
4) user is NOT Admin and wants to mount a volume[1] to a drive letter -> Not possible
5) user is NOT Admin and wants to mount a device to a drive letter -> OK, granted
6) user is NOT Admin and wants to mount a device that does not ALSO expose a volume (such as an IMDISK device) to a drive letter -> OK, granted, see #5 above
The 6 cases above are just 4: 1,2,4,6, and in the remove verb we have the same:
1) user is Admin and wants to unmount a volume[1] from a drive letter -> OK, granted
2) user is Admin and wants to unmount a device from a drive letter -> OK, granted
3) user is Admin and wants to unmount a device that does not ALSO expose a volume (such as an IMDISK device) from a drive letter -> OK, granted, see #2 above
4) user is NOT Admin and wants to unmount a volume[1] from a drive letter -> Not possible
5) user is NOT Admin and wants to unmount a device from a drive letter -> OK, granted
6) user is NOT Admin and wants to unmount a device that does not ALSO expose a volume (such as an IMDISK device) from a drive letter -> OK, granted, see #5 above
If we use your posted image:
The E:\ (please notice the ending backslash ) is a mount point to a device (\Device\HarddiskVolume3) that *something else* (the MountManager) already mapped to a volume.
The C:\ is the same.
The R:\ is a mount point to a device (\Device\Imdisk0) that flew over (or below) the MountManager and thus it is NOT mapped to a volume.
The X:\ is a mount point to the SAME device \device\harddisk0\partition1 that - by another name (\Device1HardDiskVolume1) was already intercepted by the MountManager and assigned a Volume (but NOT a driveletter/PathName).
So the X:\ is connected to the \device\harddisk0\partition1, now, can an Admin assign another driveletter (let's say Z:\ for the sake of the example) to the Volume \\?\Volume{e26e7b15-122a-11e7-82bf-806e6f6e6963}?
Can he/she assign the SAME driveletter X:\ to the Volume?
IF this is possible, then there would be the *only* case where there would be (in theory) a difference between removing the drive letter (a SAME drive letter, in all other case the drive letters are different) from the device OR from the volume.
Otherwise the "identifier" is a driveletter, and we can use (sometimes I like it) the English passive form, i.e. instead of saying to the program in gentle and exact terms:
please remove this drive letter from the device it is connected to
OR
please remove this drive letter from the volume it is connected to
we can tell it:
have this drive letter removed (implied, the grumpy: I don't §@ç#ing care how you do it or to what the heck the drive letter is connected to, just get rid of it )
Wonko
[1] I believe that a volume (if it exists) MUST be connected (by the Mount Manager) to a device, or, if you prefer, a device can be "only" a device or "both" a device and a volume, or - maybe even better - a device can be called by its (various) names or - if connected by the Mount Manager to a Volume, by its volume name, but volumes without an underlying device do not exist.