Sometimes when you are working with the Qlik Sense APIs you need access to information that is not connected to the Qlik Sense document you are working with, but rather to the QIX Engine running.
You find the apis for this in the global object in the Capabilities APIs, or in the Global class in the Engine API. While the Capabilities APIs only include very few methods, the full list of calls in the Engine API is much longer.
Finding the Global object
While there is a method in the Capabilities API to get the global object, in most cases you should not use it. In fact if you are building an extension you never should. Instead you should use the global property on the app.
That is because the getGlobal() method will open a new web socket to the engine, while app.global property will use the web socket used for the app. There is not reason to open another web socket to the server when you already have one, and in an extension there always is one.
The only situation getGlobal() should be used is when you do not have an open web socket, like in a mashup before the app is opened. And in that case you might be better off with a REST call.