Jr and Sr's refers to those enrolled in the Platoon Leaders Class program (PLC). They do a 6 week increment for 2 summers (during college) rather than one, 10 week Officer Candidates Class (OCC). Both programs make new 2nd LT's, but PLC is easier to be accepted to and I believe you receive time in grade from your first day at OCS for doing that program...So you'll makes more money as a newly commissioned 2LT than a regular OCC.