Can fly passengers in 757 etc as a co pilot with CPL but you require an ATPL to captain 757 etc.
A CPL can command a/c under 12,500lbs this includes the king air, citation 1+2. All jet a/c must be flown multi crew for public transport but a/c under that weight the captain can be a CPL.
If ops manual or CAA regulations require multi crew this can count towards 500hrs multi crew requirement.