What does the 6 stand for in interstate 635?

Answer 1

In general, if the first digit of a three-digit Interstate highway number is even, it denotes a bypass or loop. If it is odd, it denotes a spur.

I-635 does indeed follow that convention. The number is used twice; once as a partial loop around Dallas, Texas, and again as a bypass around Kansas City, Missouri, although it does not connect to I-35 at both ends of the bypass, in either case.