\section{RPID Features}
\label{sec:additional}

Below, we summarize and motivate the major additional features that
RPID adds to PIDF.

The PIDF definition does not clearly describe what a \xmlelement{tuple}
represents.  We add an \xmlelement{class} element
(Section~\ref{sec:class}) that allows a presentity to label tuples in
ways that make sense to the presentity, e.g., to group similar tuples by
name.  The \xmlelement{contacttype} element describes whether a tuple is
a device, a set of devices with a common address (``service''), a human
face-to-face contact or a presentity.

While the PIDF definition describes which means of communications
are available for a presentity, it does not describe the activity that
the presentity is currently engaged in. The \xmlelement{activity}
(Section~\ref{sec:activity}) element adds this information. 

The \xmlelement{idle} (Section~\ref{sec:idle}) element indicates when
the device was last used or simply whether it has been idle.

To help the watcher gauge the appropriateness of different types of
communications, we indicate the type of place the user is currently in,
via the \xmlelement{placetype} element (Section~\ref{sec:placetype}) and
hint at the privacy available via \xmlelement{privacy}.

PIDF defines a \xmlelement{timestamp} element indicating the date and
time of the status change of a tuple.  RPID adds a validity period for
\xmlelement{activity}, \xmlelement{placetype} and \xmlelement{privacy}
values, as a hint how long the current status is likely to be valid.

An important sub-case is that a presentity is interruptible only under
unusual circumstances, after mediation by some, typically human,
authority such as a secretary or supervisor.  We allow the presentity to
convey that certain contact addresses actually belong to a different
person, presumably one that can either interrupt the presentity or
otherwise assist.  The \xmlelement{relationship}
(Section~\ref{sec:relationship}) element allows to indicate that a
particular tuple refers to a different principal or presentity.

Note that this document does not defined a new content type.  Rather, it
inherits the content type from \citenorm{draft-ietf-impp-cpim-pidf},
namely \verb|application/cpim-pidf+xml|.
