[ODE] How to use category bits?

[michael kapelko]

I defined ray's category bits. And collide bits define with which geoms 
this geom will collide. As I see, default bits are 0xFFFFFF or smth. So 
ray has 0xFFFFFF collide bits, 0x000002 category bits, therefore it 
can't (shouldn't) collide with 0x000001 which is of capsule. 0xFFFFFF != 
0x000001, is it? Or am I misunderstanding smth?
BTW, what does "~" mean in "~2"?
Thanks.