A subnetwork is a loosely-defined term in Applied Energistics 2, but it generally refers to any smaller ME system that supports your main network or handles a specific task. They are typically small enough to not require controllers.

• Restricting Access – Prevent certain devices from accessing your main storage. For example, an import bus on a pipe subnet should not have access to your main cells, or it will dump items into them instead of the destination inventory.
• Saving Channels – Reduce channel usage on your main network. For example, a single Pattern Provider can output to an Interface connected to several Storage Buses on multiple machines, using 1 channel instead of one per machine.

Colored cables have nothing to do with subnetworks except that different colors prevent cables from connecting to each other.

• Item/Fluid Transfer – An Import Bus and Storage Bus acting like a pipe system.
• Filtered Breaking – Annihilation Plane connected only to a Storage Bus, ensuring the plane can only place items into a specific inventory.
• World Placement – Interface feeding a Formation Plane, so whatever enters the Interface is placed or dropped into the world.
• Automation – Controlled setups like automatically creating Certus Quartz, regulated with an ME Level Emitter on the main network.
• Specialized Storage – A farm output system isolated via a Storage Bus on an Interface, preventing farm overflow from filling main storage.

A very useful tool is the Quartz Fiber. It transfers power between networks without connecting them, allowing you to power subnetworks without needing extra Energy Acceptors or power cables everywhere.