A property is set to null if its definition does not set a default value. For example:

<property name="favoriteColor" data-type="string"/>

You can also explicitly set a property’s default value to null as follows:

<property name="favoriteColor" data-type="string" default="__NULL__"/>

This technique is useful if, for example, you combine two or more repository definition files into a single template and need to override a non-null value and restore the default to null.