An ATX casing is the standard size computer case. Most of the computer cases fall under this category. This kind of computer case is also called a midi tower. You can place an ATX motherboard in this case, so you place more expansion cards than in a smaller casing. For example, you can build a complete gaming PC with an ATX casing or a computer that you place on or under a desk in an office setting. The case is also more future-proof.