prerequisites of pull_up_sublinks

I'm reading the pull_up_sublinks, and find the below comments.

 * However, this optimization *only*
 * works at the top level of WHERE or a JOIN/ON clause, because we cannot
 * distinguish whether the ANY ought to return FALSE or NULL in cases
 * involving NULL inputs. Also, in an outer join's ON clause we can only
 * do this if the sublink is degenerate (ie, references only the nullable
 * side of the join).

I tried to write some SQLs but still can't understand the above comments. Any
help here?
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