Its for purposes of fixing stuff.That means you don't get +50% to engineering permanently,it only means that when you repair something it will count as if you had 50% more engineering.For example if you had 100% engineering and go fix something you would increase condition for like 7%,but when you take one mr.fixit perk it will count as if you had 150% engineering(because you have 100% basic and mr.fixit gives +50%) and you would repair condition of item by 12% instead of those 7%.