It is PCIex1 slot.  It is used for low speed add in card.  #21 on diagram.    a PCIe x1 slot is used to plug in low demanding PCIe expansion cards that do not have a very high throughput (transfer rate).  In 8940 it is a gen 3 PCIe.  The Gen 3 x1 slot has a throughput of about 1 GB/s. The following are some of the cards that use the PCIe x1 slot. notice there is also a PCIex4 slot at bottom of mobo, #19. [IMAGE]  
Between the GPU and CPU on this board there is an input called "Slot 1".  What is this for?   [IMAGE]
8940 Motherboard - What is "Slot 1" - Picture included