Two types of gunners for US. Reload (REL) and Accuracy(ACC) You can choose at each stage which to focus on. So for your first step above armament sailor you could choose accuracy(ACC) then at the next step choose reload(REL) and then accuracy for the 3rd step. that is what is meant by ACC/REL/ACC
Whatever you choose, either accuracy or reload gets the biggest ability boost for that stage. Hope this helps.