The Stormtrooper armor was sectional, and when removed, you could access the inside of each piece. I believe the segment you are looking at covers the emergency air supply and some of the power/electronic equipment. (It's listed as the 'Backplate'.) Those aren't meant to be markings (Rank markings were indicated by a colored pauldron), but instead protrusions due to having to accommodate what was underneath of them.

Incidentally, the tube below is a custom thermal detonator, with a secret trigger code known only to the individual trooper. (Star Wars Official Fact file, volume 4.)

Most of this information comes from Star Wars: The Visual Dictionary, as referenced by Wookieepedia.