An NSI Aggregator can take a single request to provision across X domains and translate it into NSI requests across each of those domains. This prevents the requestor for having to handle requests and state across all of the different domains involved with their request.
NSI uses a tree structure instead of a chaining structure (like IDCP) which allows the aggregator to better handle errors from errors in other domains. This prevents a single NSA from causing all of the other NSA to get into a failed state.
Below is an example diagram of an NSA aggregator handling the request from a requestor and proxying the request to the provider agent. This shows the request making its way from the reuqester through the Aggregator and to the provider agent. It also shows what happens if there is a SOAP fault between the provider and the Aggregator and how this gets filtered down to the requester.