There actually is a high-choice, mid-choice, and low-choice grade. High choice is usually what good (not great) restaurants use, or steakhouses that don't designate their beef as prime (i.e. Outback Steakhouse).
Regardless, unless the meat is graded as prime, you're not really technically getting prime rib. However, the term prime rib is used to describe the particular cut of beef (standing rib roast), and not just the grade. The USDA actually recognizes this and the standing rib roast is the only cut of beef that can be called "prime" even though it might not be.
"Boldness has genius, power, and magic in it."
Johann Wolfgang Von Goethe