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ROM-based drivers are stored in expansion ROMs. Disk-based drivers are located in the Mac OS file system, in the Extensions folder.
ROM-based drivers with the correct information in their driver description structures are installed and opened by the Macintosh firmware, acting as the driver's client. These are the only drivers available at the beginning of system startup.
Disk-based drivers are located and opened as needed. Once the file system is working, Mac OS can replace outdated ROM-based drivers with disk-based drivers. Experts that control disk-based drivers locate and initialize their drivers soon after. Drivers that are disk-based but not under expert control, and that are not needed by Mac OS during startup, remain uninitialized and unopened until a specific client requests access to the device associated with the driver.