xcb vs xcutsel?

[Dan Stromberg]

What exactly is the relationship between xcb and xcutsel?

It seems like I can almost always use xcb is a convenient way of doing
file transfers (for instance, if I've already tunneled X through
multiple routing discontinuities), but once in a while, the distinction
between the cut-and-paste and xcb -p/-s gets blurred - usually due to
running xcutsel, but sometimes it seems to just happen spontaneously.

The truth of the matter, or even a lead that might eventually produce
the truth of the matter, would be greatly appreciated.

Thanks!