A RSCA is a figure set as the standard of an eval by the reporting senior. So if the Sailor is graded a 4.2 and their reporting senior's RSCA is 3.7, they scored .5 above the average. This matters mostly for E6 that is attempting to make board for Chief, as its a major contributor towards a value that sets the cutoff for those that can be eligible for board based on the exam and those that don't make the cut that year, no matter how good their overall body of work is. AD is very exam orientated, rate dependent, for advancement.
As for packages, not sure it really matters at that point. Everything I've heard is your AD work (evals, collaterals, etc.) is expected to be exceptional and if it is anything but then it just goes against you.