sentences:  Audi "RS" models are considered direct competitors to similar sized hardcore sport models from BMW M and Mercedes-AMG, whereas the Audi "S" models (as of 2010) have been positioned more as engine upgrades.Taking the 2012 model year for example, the Audi "RS 5" competes directly with the BMW M3, M4 whereas the Audi S5 competes with the BMW 335i, while the Audi RS 6 competes with the BMW M5 and the Audi S6 competes with the BMW 550i.However the Audi "RS 6" (C6) was never exported to the United States due to emissions regulations, leaving the Audi S6 (C6) as the top performing trim to compete against the BMW M5 (E60) in that market 2013 Audi S7.