Subnetworks

"Subnetwork" is a rather loosely-defined term, but one might say that a subnetwork is any network that supports your main network or does some small task. They are typically small enough to not require controllers. Their main 2 uses tend to be:

To restrict what devices have access to what storage (you don't want the import bus on a "pipe" subnet to have access to your main net storage, or it will put the items in your storage cells instead of in the destination inventory).
To save channels on your main network, like having a pattern provider output to an interface connected to several storage busses on several machines, using 1 channel, instead of putting a pattern provider on each machine, using several channels.

Very important in making a subnet is keeping track of the network connections. Often, people put together some jumble of interfaces and busses and stuff and expect it to be a subnet when all the devices are still connected to the main network through various fullblock devices.

Cables with different colors have nothing to do with making a subnetwork other than that they won't connect to each other.

Some examples are:

A setup to replace your Trash Can/Void Upgrade with an entire AE2 network that decides how to best utilize your garbage. Intelligently route items to a composter array or some modded recycler depending on availability and demand.
Build abstractions. Manage all the details of a complex crafting operation from your subnetwork, so from the perspective of your main net, the entire factory "looks like" one machine.
Parallelism. Replace a slow machine with 10 copies of the slow machine. From the perspective of your main net, nothing's changed, and you aren't even using any more channels.
An import bus and storage bus set up to transfer items or fluids from one container to another like an item or fluid pipe.
An annihilation plane and storage bus, so that the only place the annihilation plane can put what it breaks is the storage bus, allowing you to filter the plane.
An interface and formation plane, so that whatever is inserted into the interface gets pushed to the formation plane and placed/dropped in the world.
A setup to automatically make certus quartz, regulated and controlled by a ME Level Emitter on the main network.
A specialized storage system accessible from the main network via the special storage-bus-on-interface interaction, in order to store the output of a farm without endlessly overflowing your main storage.
And so on

Very useful for making subnetworks is the Quartz Fiber. It transfers power between networks without connecting them, allowing you to power subnets without needing to put energy acceptors and power cables everywhere.