My interpretation of the Battlebots rules is that a Multibot [their word for cluster] is not out until it has lost over 50% of its units by weight. They are not explicit on that and I'll ask them to clarify but I think that is their practice. For example their rules specify: 'If no single Multibot segment exceeds 50% of the total eight of all segments, a sticker will be applied to each segment indicating that segment's weight.' and 'If a competitor robot is a Multibot, judges must be informed, prior to the match, of the weights and descriptions of each of the segments.'
We saw that in the most recent series when Witch Doctor was immobilized the match was over, even though their second [smaller] robot, Shaman was still fighting. But in the Warrior Clan match against Nightmare, the total destruction of one of the Warrior Clan minibots did not stop the match and they went on to win.