What's in a name?
5.11 is a rock climbing difficulty level as listed in the Yosemite Decimal System.
With skill levels ranging from 5.0 (easy) to 5.10 (difficult), 5.11 is even more grueling. 5.11 is officially defined as: “After thorough inspection, you conclude this move is obviously impossible; however, occasionally someone actually accomplishes it.”
This explains 5.11 tactical's view to clothing design. They make them comfortable, easy to look after and tough as old boots - no wonder they've been endorsed by Ray Mears!