att.declarable provides attributes for those elements in the TEI header which may be independently selected by means of the special purpose decls attribute. [15.3 Associating Contextual Information with a Text]
Moduletei — The TEI Infrastructure
Membersavailability bibl biblFull biblStruct broadcast correction correspDesc editorialDecl equipment geoDecl hyphenation interpretation langUsage listApp listBibl listEvent listNym listOrg listPerson listPlace metDecl normalization particDesc projectDesc punctuation quotation recording refsDecl samplingDecl scriptStmt segmentation settingDesc sourceDesc stdVals styleDefDecl textClass textDesc xenoData
defaultindicates whether or not this element is selected by default when its parent is selected.
Status Optional

<rng:ref name="data.truthValue"/>
Legal values are:
This element is selected if its parent is selected
This element can only be selected explicitly, unless it is the only one of its kind, in which case it is selected if its parent is selected. [Default]

The rules governing the association of declarable elements with individual parts of a TEI text are fully defined in chapter 15.3 Associating Contextual Information with a Text. Only one element of a particular type may have a default attribute with a value of true.