The first two there are “hidden” partitions, used for system stuff and recovery (storing the install image of Windows, so they don’t have to give you separate media like DVDs). Only one (the OS one) will have a drive letter and will be actively used and accessed day to day by the user. Hence it is the only one that the OS sees, and will be the only one that would need any kind of backup.
The original comment would relate to having two “active” partitions on the same physical hard drive, such as an OS partition but also a data partition. It’s quite common to set Windows up that way (I often used to do it when installing earlier versions) as that way if you need to update or reinstall Windows then you can do it on the OS partition without having to worry about the safety of the data.
Of course newer versions of Windows make that less easy as they can themselves mess around with partitions and screw things up, but that’s a side-rant.
