The application library
As previously mentioned, the Assets panel can show object types and fields. However, it has a third tab for predefined library entities. If you click on this tab, you will see the application library.
The library contains predefined entities that can simplify the Qlik Sense usage for a business user. Dimensions, measures, or entire visualizations can be stored in the library.
You do not need to use the library—nothing has to be predefined for Qlik Sense to work. However, if you want to reuse formulas or you have a situation where your task is to deliver an app to a business user, it is a good idea to use the library.
Which fields should be exposed?
Often, you have many fields in an app, of which only a few should be exposed as dimensions. Then, you should use the library to define the fields that are appropriate as dimensions and name them in a way that they can be easily understood.
A dimension can also be a group of exposed fields, a drill-down group, or a cyclic group...