In a nutshell, disk shares are the contents of each individual disk. The don't show up on the network by default, but you can turn that option on if you want. User shares are the combined view of all identically named root folders on each of the individual disks. The reason disk shares aren't shown by default is that it's easy for an inexperienced user to corrupt files if they combine file operations between the two different schemes. Until you figure out exactly how disk and user shares interact, don't mix them in file operations. Copy or move only disk to disk, or user to user, never disk share to user share. After you have a handle on exactly what's going on behind the scenes, it's easy to avoid the pitfalls and you can navigate as you please between the two.
Novice users often see their files in two different named locations and think they accidentality created duplicates somehow, or try to take shortcuts with file management resulting in corrupting their files.
Quick example. /mnt/disk1/shareA/Subfolder1/text.txt is /mnt/user/shareA/Subfolder1/text.txt, so deleting the first file makes both go away. If you instead manually create /mnt/cache/shareA/Subfolder1/text.txt so the same folder and file name are on both drives, then the user share named shareA will only show one of the two files, and if you delete the file visible in the user share, it will delete one of the files, and nothing will appear to happen because the user share will start showing the other file. Once you understand the nuances, you can do some interesting tricks with it.