\subsection{Label Element}
\label{sec:label}

% http://www.w3.org/TR/xmlschema-0  4.2/4.3

The \xmlelement{label} attribute is used by the presentity to label
tuples.  The value is chosen arbitrarily and {\MUSTNOT} be modified by a
composing server or PA.  There is no requirement that all tuples within
a presence document differ in their label or have a label at all.
Typically, the label remains the same across subscriptions and across
watchers.

\motivation{The \xmlelement{label} makes it easier for policies to
operate on presence documents.  The 'id' \xmlelement{tuple} attribute is
not guaranteed to remain constant across subscriptions.  The PIDF
specification does not prevent a PA from modifying the 'id' attribute. 
An element, rather than an attribute, was chosen since it appears less
likely to cause interoperability problems with plain PIDF parsers.}

