Actually, if you look closely at the silk-screening on the board, it actually does indicate which slot is on the NB and which slot is on the CPU....Butttt, then the numbering is not same as it is in the BIOS where you enable that feature, so I sort of had to draw a diagram for myself to figure it out.Yes! the PCIE breakdown diagram in the motherboard manual is super important to match which slots get split and you need to be careful because it is possible that the onboard SAS is configured against lanes that can be bifurcated on another slot and may present you with an option like x4x4x4x4 .
This is doubly true (ha ! Punny!) on an X9DRD-7ln4f using bifurcation as onboard devices are split between CPUS and devices at least on mine and the bios revision I am pretty sure I have an option to bifurcate the lanes going to the onboard 2308 selecting x4x4x4x4... Obviously you don't want to do that.
This is my diagram here: https://photos.app.goo.gl/YZ1oAsPrNphEMHxM9
I don't know if you can make sense of it, but when you go to the settings in BIOS, it should immediately make sense.
-JCL