I'm new in WoW and I have a question I can't figure out on my own.
What's the meaning of the arrows under some (but not all) upgrades on a ship?
Example: my Phoenix has upgrades that lead nowhere (apart from improving the ship of course), whil it has two hull upgrades that directly lead, with an arrow, to the Omaha Tier 5.
What's the meaning? Does that mean that I HAVE to buy those two hull upgrades in order to research the next ship in the line? And what about ship experience? If the Omaha costs 17500 exp, and the two hull upgrades cost 1600 and 2500 respectively, does it mean that the true cost of the Omaha is 17500 + 1600 (mandatory) + 2500 (mandatory)? Or that 1600 and 2500 get cut off the original cost of the Omaha, so that they both count towards the next ship AND improve the Phoenix while at it?
What am I missing? please help.