My 2014 X351 L Portfolio went for an inspection, my mechanic sent me this picture and mentioned "cable where it connects from rear to front and boosting completely corroded off."

I would like to understand what is this cable and where it is located. Any part numbers which can help. Any tips on how I can replace it myself would be helpful

 

Sounds like those are the cables that run from the battery to under the hood where the positive and negative connections are located. Those connections are not for jumping the car, but people don't know that and use them that way. They are low amperage connections for slow charging the battery or providing emergency power when the battery dies to be able to operate the truck, windows, etc. I don't know where those corroded connections are located, but you can follow the wires.

 

Those are under the car on the passenger side, next to the transmission tunnel. They are hidden by the plastic undertray panels. I think the name for the junction in your wiring diagrams is "C3DC11A" and "C3DC11B". I can't find a part number for this, I would replace the corroded studs with new hardware.