Another useful factor that can help us draw module boundaries is application layers. A typical application involves layers or tiers of functionality like the UI layer, business layer, data layer, and so on. That's a great way to draw some initial module boundaries before then looking at other categories to modularize further. A module should ideally not be a part of multiple application tiers. Sometimes, different tiers are deployed on different physical hardware. So, having tier separations guide module boundaries make logistical sense as well when it comes to deployments.