Guess I should have tried to be more informative, sorry. Yes, I am aware that a particular tail number aircraft can (and probably will) go elsewhere during its daily (or weekly) legs. That is why I stated that MSP-PVG pieces will (or might) be covered by a combination of DTW-PVG, LAX-PVG, LAX-HND, MSP-PVG interactions. So since (in the example) there are 4 round trips (8 legs), and by looking at Deltas schedule, there will be somewhere between 4 and 8 aircraft doing the segments at any given time. Based on current Deltas schedule of A350-900XWB flights, there are 14 legs (7 round trips) being done by 11 aircraft, all of which are daily legs.There will be 4 more legs being added soon for 2019 and only 2 additional A359 aircraft being added (18 legs - 9 round trips - 13 aircraft). At this point (of what I'm aware of), Delta is adding only 2 legs - 1 round trip - 2 aircraft for the 2020 calendar year. I can then venture a guess that 1 of the 2 A/C being added in 2020 will cover the MSP PVG round trip and the other can then be used as a spare (I do understand that the 15th A/C will not be the same tail number day after day, but 1 A/C will remain idle at any given time to be used somewhere within the 20 legs if a problem occurs in the 14 being used.