Look at this example. Its made with the old version of RestAPI first made by @thomasaull years ago.
To get the context, you can navigate to https://kingspark.fr and https://valideur.mykingspark.fr . I am speaking about a system which give the possibility to some of our client use their mobile or a barcode scanner device to give "free of charge parking" of their customer. Its composed with custom hardware, software and devices or mobile apps (you see it on GooglePlay).
Image #1: List of Parkings // Image #2: The custom made SAGAS Terminal // Image 3: A configured User / Device available for registration and use.
On the first picture, you can see a list of "Parkings", and some can have the "Valideur" functionality. We can configure attached devices and some users with specific role to get access for registration.
And then, imagine the following. A customer land on the parking, grab a ticket and go to their rendezvous At the end, the guy in the office use his "Barcode Scanner" to "validate" the ticket of the customer; This mean that the customer will not pay anything when landing on the terminal to exit the parking, and when he will present the ticket on the barcode-reader installed on the terminal (the "black hole" you can see on the center in the picture), it will be recognized by another software and thought a call on ProcessWire Rest API backend.
I made you two screencasts, the first is the software which once installed and registered, get the quota (number of validation available) from the user assigned to the license-code, and the second is my mobile which get notification sent from ProcessWire (by this module) from a monitoring system to get real-time information on registration.
If you have any question, do not hesitate. There a more example, but you should get the whole idea ?