Microsoft provides a general-purpose monitor class function driver, Monitor.sys, that handles most monitor-related tasks. There is no need for a vendor-supplied monitor driver unless the vendor wants to provide services beyond those provided by the monitor class function driver.
If a monitor vendor chooses to provide a filter driver, that driver is represented by a filter device object that sits above the functional device object in the monitor's device stack. The filter driver handles requests from user-mode applications, also provided by the monitor vendor. The interface between the filter driver and the user-mode applications is private and known only to the monitor vendor.
Note that programmatic control of a monitor through the Display Data Channel Command Interface (DDC/CI) is not handled by the monitor device stack, so monitor vendors should not write filter drivers for that purpose.
For a representation of a monitor device stack, see Monitor Class Function Driver.