A plus or minus is often added to the end of a rating. Plus meaning harder and minus easier. The length of the fall and the injury potential are combined and interpreted to give the overall rating of the pitch. Beware when climbing hard aid in the desert, the rock changes and an A2 route can quickly become A4. On the other hand climbs that were A4 twenty years ago are now A2 with modern gear.
Clean aid refers to routes that can be climbed without a hammer.
C1 is the easiest aid, usually camming devices and nuts are sufficient. C2 may require some trickery, such as hooking or dicey Tri-cams in pin scars. Other tricks include hand placing pitons, knotted slings, trusting just two cams of a friend, Lowe Balls stacked on Knife blades, etc. Some losers believe that cheater sticks are part of Clean aid- cheaters are losers and cheaters never win. The best thing to remember about an aid rating is that one man's A5 is another man's A1.