I would bet decoration, possibly with some gender or social display meaning. Almost all salarians wear clothing that either emphasizes the chest ridge (bottle opener style) or it draws the lines of the outfit to the chest ridge either in an straight bar (Commander Rentola, Captain Kirrahe, Captain Kirrahe) or an bracketing pattern (Chorban, Ish, Schells).
A couple of possible explanations:
- An anatomical vulnerability. It seems most of the bar pattern are
evident in armor, so it is possible that the ridge is a place where a
minor injury can to major damage.
- A mating display of some kind. Because of their shortened life spans they need some way to indicate sexual availability quickly and succinctly. Perhaps the Bracketing pattern expressing availability and the bar and bottle opener style expressing the opposite.
- A fashion trend. It could jut be that one major salarian fashion designer takes cues forom the open vest styling of Han Solo, and another takes inspiration from old human Power Girl comics.