This is a way of scaling the master domain concept to a large network, in which there are too many user accounts to fit into a single master domain. The complete trust model certainly sounds good. In this case, it turns out to be another one of those things that seems better in theory than in practice. The complete trust domain model usually ends up being an administrative nightmare. Unlike the master and multiple master models, there is no hierarchical organization to the complete trust.
Every domain has two one-way trust relationships with every other domain in the network.
User accounts can be located in any domain, as can resources. As the number of domains increases, this model becomes unwieldy and difficult to manage. Upgrade if necessary. Check requirements for compliance with standards and regulations. Diagram the network.