The US Special Operations Command (US SOCOM) issued a solicitation for the procurement of SOF Combat Assault Rifles (SCAR) on October 15th, 2003. This solicitation requested a new combat rifle, specially tailored for the current and proposed future needs of the US Special Forces, which are somewhat different from latest generic US Army requirements, which are being fulfilled by the newest Heckler-Koch XM8 assault rifle. The key difference in basic requirements between XM8 and SCAR is that, while XM8 is a single-caliber weapon system, tailored for 5.56x45mm NATO ammunition, the SCAR should be available in various different calibers.
Ref. https://www.world.guns.ru/assault/as70-e.htm