2.1.19 [W3C-P3P1.0] Section 3.3.7, The DATA-GROUP and DATA elements
The specification states:
<DATA-GROUP> describes the data to be transferred or inferred base base URI ([URI]) for URI references present in ref attributes. When this attribute is omitted, the default value is the URI of the P3P base data schema (http://www.w3.org/TR/P3P/base). When the attribute appears as an empty string (""), the base is the local document. <DATA> describes the data to be transferred or inferred ref (mandatory attribute) URI reference ([URI]), where the fragment identifier part denotes the name of a data element/set, and the URI part denotes the corresponding data schema. In case the URI part is not present, if the DATA element is contained within a DATA-GROUP element, then the default base URI is assumed to be the URI of the base attribute. In the other cases, as usual, the default base URI is a same-document reference ([URI]). Remember that names of data elements and sets are case-sensitive (so, for example, user.gender is different from USER.GENDER or User.Gender). optional indicates whether or not the site requires visitors to submit this data element to access a resource or complete a transaction; "no" indicates that the data element is not optional (it is required), while "yes" indicates that the data element is optional. The default is "no." The optional attribute is used only in policies (not in data schema definitions).
All Document Modes (All Versions)
The following variations apply:
Only the explanatory text for each DATA-GROUP element that is present in a DATA element is displayed.
The ref and base attributes of the DATA-GROUP element are not supported. The data types are not validated against the base URI.