  Let $\Lambda$ be the collection of all probability distributions for
$(X,\widetilde{X})$, where $X$ is a fixed random vector and $\widetilde{X}$
ranges over all possible knockoff copies of $X$ (in the sense of
\cite{CFJL18}). Three topics are developed in this note: (i) A new
characterization of $\Lambda$ is proved; (ii) A certain subclass of $\Lambda$,
defined in terms of copulas, is introduced; (iii) The (meaningful) special case
where the components of $X$ are conditionally independent is treated in depth.
In real problems, after observing $X=x$, each of points (i)-(ii)-(iii) may be
useful to generate a value $\widetilde{x}$ for $\widetilde{X}$ conditionally on
$X=x$.
