I am reading about the charged current weak interaction. The notes I am using state that, with the charged current weak interaction, transitions across quark generations are possible, but suppressed. They then state that the Feynman diagrams (modulo crossing symmetry)
$d \to W^- + u$,
$s \to W^- + u$,
and $b \to W^- + u$
are all possible. There are a couple of things about this which I am unclear about.
Firstly, I'm not too sure about in which sense these are "suppressed". Can anyone shed any light on this?
Secondly, I am not sure how to interpret the wording above. Are these three simply examples of vertices which are allowed, or is this a list of absolutely all of the vertices which are allowed?