The FeatureDefinitionBaseType is the abstract base type for feature definitions. A feature definition holds that portion of feature nominal information which can be shared among several features, e.g. feature size.
The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="FeatureDefinitionBaseType" abstract="true"><xs:annotation><xs:documentation>The FeatureDefinitionBaseType is the abstract base type for feature definitions. A feature definition holds that portion of feature nominal information which can be shared among several features, e.g. feature size.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="FeatureBaseType"/></xs:complexContent></xs:complexType>