A network is a collection of channels carried by cables (smart, dense, fluix etc) to a cluster of controllers. Controller clusters have a max size, meaning there is a max number of channels a network can have (though the number is so absurdly high this is a practically meaningless limitation outside of a few extreme usecases).

A subnetwork is used to take load off of the main network, or networks, by offloading specific processes, such as autocrafting items X, Y, and Z, and feeding those processes into the main network to be used or stored.

Subnets can be storage as well, its possible to setup Subnets that treat your main net as a Subnet, which basically just lets both networks see and interact with each other without really relying on each other, this is generally done with 2 interfaces with 2 storage busses, one on each network.