The abstract global FeatureDefinition element is the head of a substitution group. Any element that is a member of the group may be substituted for the FeatureDefinition element anywhere the FeatureDefinition element could be used in an instance file if it were not abstract.
The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:element name="FeatureDefinition" type="FeatureDefinitionBaseType" abstract="true"><xs:annotation><xs:documentation>The abstract global FeatureDefinition element is the head of a substitution group. Any element that is a member of the group may be substituted for the FeatureDefinition element anywhere the FeatureDefinition element could be used in an instance file if it were not abstract.</xs:documentation></xs:annotation></xs:element>