meaning of grid numbers in css

[smallcompany]

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.

[not2easy]

The classes shown above have no meaning outside of the framework you are working with, I would look at the documentation they offer for that grid. The css copied above shows a list of various classes that are assigned the properties that follow them as you described. You can look at the properties of any parent elements to see what properties they have that might affect widths.