Yes it can be a bit confusing... you see your system has two different drive connections the older SATA (HDD) and the newer PCIe blade (SSD), so you could have one or two drives installed.
Now the bad news! If your system was only a HDD model the second drive port is not present! Which means you can only use the SATA port, swapping it out for a SATA SSD. Here are the guide you'll need to review:
The last thing here is the 'Thin Series' iMac's are a bear to open! And if you don't use the correct tools and techniques you can damage the display assembly which would be an expensive Opps! So make sure you're upto doing this. If not find someone who has done a few and let them do it.