Set true when this subtype should be selected by default if no other subtypes are selected explicitly. Note that a subtype with this parameter being true will not be shown in the subtypes list.

Must be a boolean value, either "true" or "false".

This may also be a reference to a resource (in the form "@[package:]type:name") or theme attribute (in the form "?[package:][type:]name") containing a value of this type.

[Android Documentation]