In JOSM there used to be two options when downloading members: download members and download incomplete members.
Some time ago a third option appeard: “Download with members”.
What does this do different than “Download members”?
Download with members: Download the current version of the relation and the members (as the server knows it)
Download members: download the members of the relation as JOSM knows it (doesn’t update the relation)
In the past “Download members” did what “Download with members” does now.
Typically “Download with members” is the better option as it is faster.
“Download members” may be useful when working with reverter / undelete
Thanks!