The Source Data section is confusing.
There is a User Data, a Computer, and a Source Data sections. It is not totally clear which does what.
The Computer section is a traditional folder/file browse area, showing your drives, folders under each, etc.
The User Data section are shortcuts to certain folders in the Computer section. For example you can check “Desktop” in the User Data section and it will basically select C:\Users\username\Desktop for backup.
The Source Data section is a listing of what things you’ve selected for backup in the User Data / Computer sections. It allows you to quickly deselect something for backup that you previously selected without having to find it in the directory tree.