When you add or connect a new volume (disk or drive), Windows will automatically mount it with an assigned drive letter by default.
Each drive (volume or partition) will have an unique Volume GUID assigned to it by Windows. This ensures Windows can always uniquely identify a volume, even though its drive letter has changed. On systems with a lot of storage you will often run out of drive letters for your partitions and volumes.
Whenever you reconnect a drive to the computer, it will always use the same drive letter it was last assigned or changed to.
If you unmount a drive, Windows removes the volume mount point from the specified directory, dismounts the volume, and makes the volume not mountable. This means the specific drive (volume or partition) will have its drive letter removed and no longer be automatically mounted and assigned a drive letter whenever connected to the computer until you manually mount it again. This can be handy if you no longer want a disk or drive (volume or partition) to be assigned a drive letter when connected until you want to manually mount it.
When you manually mount an unmounted drive, you make the volume mountable again by creating a volume mount point with an assigned drive letter. This will allow Windows to automatically mount the drive with its last used drive letter each time the drive is connected to the computer.
This tutorial will show you how to manually mount and unmount a drive or volume in Windows 7, Windows 8, and Windows 10.