Application.config_change

You're seeing just the callback config_change, go back to Application module for more information.
Link to this callback

config_change(changed, new, removed)

View Source (optional)

Specs

config_change(changed, new, removed) :: :ok
when changed: keyword(), new: keyword(), removed: [atom()]

Callback invoked after code upgrade, if the application environment has changed.

changed is a keyword list of keys and their changed values in the application environment. new is a keyword list with all new keys and their values. removed is a list with all removed keys.