Petty officers are the three ranks above E-3, they are basically NCOs. There is the E-4 Petty officer third class, E-5 Petty officer second class, and E-6 Petty officer first class. There are of course ranks above that such as the E-7 Chief Petty Officer, E-8 Senior Chief Petty Officer, and E-9 Master Chief Petty Officer. But we are more likely to see quite a few E-1 to E-3 in the med bay with the exception of a couple of squad leader E-4 to E-6s. The E-4s through E-6s are responsible for leading squads in a company while the Chief Petty Officers would be the ones likely to take control of the platoons in the case that their Ensign is not there. But basically the Petty officer classes mean you've been in service for a little while.