When mountain climbing, what do these numbers represent?

Difficulty rating of technical rock climbing (Yosemite Decimal System).

Class 1: Walking with a low chance of injury, hiking boots a good idea.

Class 2: Simple scrambling, with the possibility of occasional use of the hands. Little potential danger is encountered. Hiking Boots highly recommended.

Class 3: Scrambling with increased exposure. Handholds are necessary. A rope should be available for learning climbers, or if you just choose to use one that day, but is usually not required. Falls could easily be fatal.

Class 4: Simple climbing, with exposure. A rope is often used. Natural protection can be easily found. Falls may well be fatal.

Class 5: Is considered technical roped free (without hanging on the rope, pulling on, or stepping on anchors) climbing; belaying, and other protection hardware is used for safety. Un-roped falls can result in severe injury or death.

    Class 5.0 to 5.15c[6] is used to define progressively more difficult free moves.

en.wikipedia.org/wiki/Yosemite_Decimal_System


OR:

Class 1: you fall, you're stupid.

Class 2: you fall, you break your arm.

Class 3: you fall, you break your leg.

Class 4: you fall, you are almost dead (i.e., you can't breath and move your arms, legs, and head).

Class 5: you fall, you are dead.

In Mountaineering, Freedom of the Hills, author Ed Peters explains the subdivisions of class 5:

"The experienced climber, having accomplished or attempted free climbs of varying degrees of difficulty in the YDS class 5 range, gains an understanding of the level of difficulty involved. To the beginner, however, these ratings are simply a set of numbers, understandably, easy if rated 5.0 and impossible if rated 5.13. To provide a slightly better understanding within the class for the beginner the following tongue-in-cheek description is provided:

5.0 to 5.4: There are two hand- and two footholds for every move; the holds become progressively smaller as the number increases.

5.5 to 5.6: The two hand- and two footholds are there, obvious to the experienced, but not necessarily so to the beginner.

5.7: The move is missing one hand- or foothold.

5.8: The move is missing two holds of the four, or missing only one but is very strenuous.

5.9: The move has only one reasonable hold which may be for either a foot or a hand.

5.10: No hand- or footholds. The choices are to pretend a hold is there, pray a lot, or go home.

5.11: After thorough inspection you conclude this move is obviously impossible; however, occasionally someone actually accomplishes it. Since there is nothing for a handhold, grab it with both hands.

5.12: The surface is as smooth as glass and vertical. No one has really ever made this move, although a few claim they have.

5.13: This is identical to 5.12 except it is located under overhanging rock."

climber.org/data/decimal.html

I've free soloed 5.5-5.6 routes, so Alex and I are pretty much in the same category.

I always thought those numbers represented the number of people who died attempting those routes. Now I know better.

I got blisters on my heels and lost two toenails a couple of years ago in Chile, so WWBF and I are pretty much in the same category.

BL, I actually stopped and got some of the scree out of my shoe. Try it next time and see if it works for you.

On a more serious note, if you've ever walked up to El Cap and stared up at its almost 3,000 foot vertical rise and glassy surface, you'll know how insane Alex's accomplishment is. The easy part is first - The Nose, which is almost vertical. Then it really goes vertical. Handholds become fingerholds, or fractions thereof.

We did a two-fer, walking tow paths on both sides.  On the Jersey side, we visited a State Park whose entrance was still swathed in police tape from being closed since Friday.  It was largely still deserted since most people who canceled their plans left them canceled. The budget deal was signed after most people went to bed.

I can find my toilet and use it in absolute darkness without mishap, 5.16 at least.

I once base camped next to a rather high Mountain and did not even feel any altitude sickness in the morning, even looked up to the top of the Mountain thinking that just maybe, just maybe one of these days I just might have to climb that mountain, if that counts. Ok nothing useful to add over here.