Poina,
The aircraft with an extra 5,000 lbs MTOGW in mode H did not have extended range aux tanks. You have described the difference (mode H/L) on your Saudia aircraft youself albeit in Kgs rather than lbs. There are a few scenarios per our MEL where you can still have a mode H W&B with the FSC in manual. Fuel has to be manually put in the tail during refuelling per the normal schedule but once airborne there is no tail fuel management. This can confuse some crews as if they see "Tail fuel management is not available" in the (o) notes of the MEL and therefore don't expect to see any fuel in the tail after refuelling. Basically with the FSC in manual they take off with the main tank pumps on and aux/tail pumps off and then turn on the aux/tail tank pumps during the after-takeoff checklist.