If the conflict.rateLimitIsAggregate option is true, then after each tick one point of conflict-credit is given to just one domain: the one at the front of the queue. If false, then after each tick each domain gets a point of conflict-credit.
In environments where it is known that every transaction will involve a set of nodes that is writable by at most one other domain, then it is safe to set this aggregate limit flag to false for better performance. (This can be determined by considering the layout of the xenstore tree and permissions, together with the content of the transactions that require protection.)
A transaction which involves a set of nodes which can be modified by multiple other domains can suffer conflicts caused by any of those domains, so the flag must be set to true.