The rules state :
1- [...] The loser must, instead, retreat. They immediately move all of their units from the region to the nearest region containing one of their control cubes [...]
2 - [...] If there are multple regions to which the loser can retreat [...] then the loser chooses where to retreat. [...]
1 - This means that you can potentially retreat quite farer than where you came from for battling ? For example, if I came from an adjacent Orbit region to battle a Surface region but my nearest controlled planet is on a planet on the opposite side of the board, my units retreat to this planet, no matter is the distance between the two planets ?
2- Do units retreat as a group like for Movement ? Or in case of equidistant retreat regions, can retreated units be splitted among these regions ?