Page 1-9 of the X9SAE-V manual shows a R/C block that references this possibility. What is R/C?
Is populate Slot 4 with anything, automatically Slot 4 & 6 will split the 16 lanes giving 8 each?
Yes. If only if user install a add on card on Slot 4, then slot 6 will only functiona as PCI-E x8. If only if no add on card install on Slot 4, Slot 6 will function as PCI-E x16.