SharePoint Features schemas
Different Features require different file formats, depending on the type of customization you are making. In most cases, a Feature.xml file contains a Feature element that uniquely identifies the Feature, specifies its scope, and points to an XML file whose top-level Elements element defines the elements comprising the Feature.
A Feature, however, might not include an Elements file, and may consist of a Feature.xml file whose Feature element, in addition to providing the previously described specifications, points to an assembly that is used in the Feature, such as in the case of declaring an event receiver.
In addition to describing the file format of Feature.xml Files and the Feature sections in Site Definition (Onet.xml) Files, this section describes the file formats used for the following Feature types: