XPS 8500 - what is the bay/slot under the Second Optical Drive bay called?

Hi,

I have an XPS 8500 to which I have added 3 HDDs utilizing the HDD cage (2 HDDs installed) and an 5.25" to 3.5" bay adapter (1 HDD installed). 

However, I have noticed an empty bay/slot under the secondary drive bay and above the HDD cage. On the chassis it is marked as F3. Does anyone know what the bay/slot under the Second Optical Drive bay is called?Front ViewManual diagramSide View