The order would be the order that the change sets appear in the history of the context (whether the context is a component in a workspace/stream or a baseline). I.e. if you were to right-click on the component/baseline and use 'Show' --> 'History', you'll always see the change sets in the same order... which is the order that the change sets were 'added' to that context (most commonly due to the 'deliver' action to a stream for example). So this is not necessarily the last modified date or creation date of a change set (although in practice more often than not this tends to be the 'last modified' date of a change set, which is really the date the change set was completed/closed - which happens during delivery to a stream - but as already mentioned, the order is not 'always' by 'last modified' date, because users can always 'resume' old change sets, or roll-back / discard/suspend change sets, etc.)