Final answer:
Devices that have expansion slots for adding new modules are said to have a modular configuration, which allows customization based on network requirements.
Step-by-step explanation:
Network devices come in two physical configurations. Devices that have expansion slots, which provide the flexibility to add new modules have a modular configuration. The modular design allows the device to be customized for different network requirements and can enhance its functionality with additional features such as firewalls, increased connectivity options, or even Power over Ethernet (PoE) capabilities. This is in contrast to fixed configuration devices, which do not have this level of adaptability and expansion capability.