This number is a recreational climbing difficulty rating from the Yosemite Decimal System, developed by climber Royal Robbins. There are five basic levels of difficulty in climbing, ranging from trail walking to Mission Impossible-style climbs. Within that, there are sub-levels, with 5.10 being the most difficult. As such, a 5.11 is considered basically impossible, yet still aspirational. 5.11 is also the name of Royal Robbins' clothing company, which started supplying climbing pants but has moved into a range of tactical gear.