Cross-network relationships enable you to spread the tasks to be performed over several individual and smaller networks thereby creating an overall network.
You link several (standard) networks with each other when you create a relationship from an activity in one (standard) network to an activity in another (standard) network.
You use cross-network relationships to link:
When you edit a standard network with cross-network relationships, you lock up all other standard networks.
When you create operative networks from standard networks with cross-network relationships, the system will ask you if you want to create the same relationships between the operative networks.
When you edit a network with cross-network relationships, you lock up all the relationships to all of the linked networks.

Even if you defined a level of detail of "one" in the standard network profile, you can still create relationships between several standard networks.
See also:
Creating Networks by Copying Standard Networks