The <f:dropdownDescriptorSelector> provides a way to define a default descriptor, e.g.:

<f:dropdownDescriptorSelector title="${%Settings file}" field="settings" default="${descriptor.defaultSettingsProvider.descriptor}"/>

This allows to define the default descriptor to be used, but does not allow defaulting to a specific instance of this descriptor e.g.:

<f:dropdownDescriptorSelector title="${%Settings file}" field="settings" default="${descriptor.defaultSettingsProvider}"/>