The Defaulter Window
To begin using Defaulter, click Window > Defaulter in the menu bar. This will open the Defaulter window. This is where you'll make all the changes related to your import settings. Changes here are saved automatically. The first time you open Defaulter, it will whitelist all your current assets, which means none of the assets you've already imported will be modified in any way.
Feature Diagram

1) Navigation Bar
The Navigation Bar is where you'll select the type of file you'd like to modify. The tab selected here, will cause the Settings Panel to show you the corresponding settings. Clicking the '!' icon will show you the "Information" panel, with more information on how to use, as well as useful links.

2) Settings Panel
The Settings Panel is where you'll change the import settings of each asset type. This panel will show you all the settings that can be overridden, and will show a greyed out version if the setting cannot be changed.
To override a specific setting, click the radio button beside the property name. The rest of the properly will become active, telling you that this property is modifyable. Modifying this property will cause it to be applied in all subsequent imports for that file type.

3) Custom Rule Selector
The Custom Rule Selector is where you can add/remove importer rules. An importer rule is a rule that will check each imported asset against the rule's conditions. If the asset meets these conditions, additional import settings will be applied to the asset. This is useful for setting up custom import settings on assets that meet unique conditions, such as:

4) Custom Rule Settings
This is the panel where you will enable/disable certain custom import rules. To enable an import rule, make sure the radio button is enabled. This will cause the rest of the associated property to become active, showing you that the current rule will check for this condition when importing new assets.