Create a logical expression
This is optional but Manifest expressions are subsequently used in the Manifest Administration view where they can be combined to define the conditional logic when a specific JavaScript file or web template should be loaded for a given UI object. Here are some pre-built expressions.
Register UI Objects (Applets)
To finalize the registration of custom files, we have to register the UI object (most probably an applet) in the Manifest Administration view.
After creating the parent record by specifying the type (e.g. "Applet") and the usage type (e.g. "Physical Renderer"), we must create at least one expression in the Objects Expressions list. The Files list is then used to associate the file which should be loaded when the selected expression evaluates to true.
To register non-applet specific files such as a custom theme.js or postload event handlers which you would like to have loaded on every view, copy one of the existing entries for theApplication type and add your custom files to a dummy expression (e.g. "Desktop").
Let's have a look at the vanilla Contact List Applet registration:
I am taking images from some other blog for reference only. You can get this topic in more details from this link.