The description of Used to communicate directly with the Service Object. in the description written on the DirectIO page you presented is for convenience only, and its implementation or non-implementation and specification details depend on the vendor.
Also, it communicates with the service object as written, and does not say that it communicates with the device (printer in case of question).
The underlying specifications are described in the UnifiedPOS specifications, which can be downloaded from here.
UNIFIEDPOS
Communicates directly with the UnifiedPOS Service.
This method provides a means for a UnifiedPOS Service to provide functionality to the application that is not otherwise supported by the standard UnifiedPOS Control for its device category. Depending upon the UnifiedPOS Service’s definition of the command, this method may be asynchronous or synchronous.
Use of this method will make an application non-portable. The application may, however, maintain portability by performing directIO calls within conditional code. This code may be based upon the value of the DeviceServiceDescription, PhysicalDeviceDescription, or PhysicalDeviceName property.
In addition, the ticket ejector ESC/POS command specification presented in the question is supported by the printer hardware and is not directly related to the POS for.NET software specification.
The POS for.NET or OPOS manual provided by the vendor should have a description of what functions the DirectIO method has, so please refer to that, or contact the vendor if there is no such description.