A meter is only a one way component from which OpenEMS read the actuall state. If you have a real meter e.g. your energy meter in your home to measure the energy consumption this can only measure and give you the value but this can’t controll something. If you need a two way communication you have to use a system like the BESS systems they are implemented.
With external simulated device I mean in this context a device that is a simulated device which is simulated external but can communicate with OpenEMS what it actually did and what it has to do. So this has a two-way communication.
This is a nature of the meter.
Have you done the Getting Started Guide before? I think this will help you to understand a little bit more how OpenEMS work and what I mean.