it raises a PR ships LUCK stat
at the final level it upgrades one of their skills
you can literally see all of this info ingame
https://i.imgur.com/QDEevum.png (embed)
DRs get more luck and TWO skills improved
i bet your next question is "what does luck do"
every 10 luck = 1% hit rate and 1% evade rate (not the hit and eva stat, but hit and eva rate which means, 100 luck = 10 of each which means a ship will always land 10% of their shots minimum and will always dodge 10% of damage taken at a minimum)
