I have heard, when you join Norwegian you enter onto the group seniority list, that is all pilots as per the deal agreed by the last Union negotiations.
It includes long haul pilots, core pilots and euro pilots.
But for long haul they have a seperste seniority list based on date of joining long haul, and from that list they work out your seniority for command etc.... Technically the core pilot would be better off joining now as he will be a year closer to upgrade.
Again this is what a mate in NLH has told me, maybe someone can confirm it.