An aggregated series is presenting data from the underlying series in the series tree. Both historic data and forecasted data is summarized from the child nodes. The child nodes for an aggregated series can be another aggregated series or forecast series on the lowest level.
An aggregated series does not have historic data, but it can be connected to a history file if you want to compare the aggregated forecast to a already summarized historic series instead of the aggregated history from the child nodes.