This is the documentation of gotify/server. Lets start with some definitions:
A client is a device or application that can manage clients, messages and applications. However a client is not allowed to send messages.
An application is a device or application that only can send messages.
Users are only able to manage (view/edit/delete) clients and applications (includes messages sent by the app) that they've created.
A message has the following attributes: content, title, creation date, application id and priority.