A few observations:
The soldier and engineer have two unknown icons with a kevlar vest, but the engineer icon has the 3 http://en.wikipedia.org/wiki/Chevron_(insignia) in it and the soldier's doesnt. I suspect the engineer has an armor buff for teammates, and soldier has something else.
Soldier's rank 5 ability interests me. At first glance it looks like c4. If it is, it doesn't look the same as the bomb we see soldiers planting on objectives in the videos. The explosive charge planted on objectives has a keypad and handles on it, whereas the charge in the icon has what looks like an antenna and the bomb sitting on a block of something, as well as a tag hanging off of it. Remote bombs? Kill-by-mail fun pack? It could also be an equipment pack, it's hard to tell.
One more thing on soldier, taken from the almighty compendium:
Pretty likely that's the shield with bullets on it.
I think maybe the soldier's one provides more armor against explosives.
I have yet to see this. Link.
http://www.fragworld.org/frag/brink/brink-compendium-update-2.html
After listing the Soldier’s “Shield teammate” ability in a thread discussing abilities on the SD forums, a dev had this to say:
“...after lots of playtesting - hit the cutting room floor so to speak, as it didn't turn out to be quite as fun as we had all hoped.”
- Richard Jolly, Media Director
You might try finishing the Compendium before you start :flamethrower: or else bliviongate: