When looking into the CSS like this:

.\35 u, .\35 u\24, .\35 u\28 1\29, .\35 u\24\28 1\29 { width: 41.6666666667%; clear: none; }
.\34 u, .\34 u\24, .\34 u\28 1\29, .\34 u\24\28 1\29 { width: 33.3333333333%; clear: none; }

What's the meaning of all the numbers prior the opening bracket?

I understand the sequence going from 1 to 12 (3 and 4 above), but \35 u\24 and all other numbers that repeat in each grid line, what's that?

I ask this as I wanted to have my own 30% width, and created the class, but now I'm not sure if something will break depending on the screen width.

How would it look like if I want to insert something between 3 and 4, so it's i.e. class u35?

Thanks

P.S.
This is from the skel framework.