What does union/intersection view in the Group View mean? I don’t see a difference.
When you select multiple groups, then union shows entries matched by one of them, while in intersection mode only entries matched by both groups are displayed.
I just tried it a minute ago, and for me the button does nothing. It always shows “intersection”, irrespective of being in “intersection” or “union” mode.
I forgot to mention that this is using JabRef 5.0.0-dev–2019-11-08----e7045c7a4, which is relatively recent.