Clarify three roles of consumers the PMIx interface

 * Clarify the three roles of consumers of the PMIx interface (clients / servers / tools)

Signed-off-by: Joshua Hursey <[email protected]>

Chap_Introduction.tex

@@ -200,3 +200,19 @@ \subsection{Attributes in PMIx}
 }
 
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+\subsection{PMIx Roles}
+
+The role of a \ac{PMIx} process in the \ac{PMIx} universe is grouped into one of three categories based on how it operates in the \ac{PMIx} environment namely as a \emph{client}, \emph{server}, or \emph{tool}.
+As a result, there are three corresponding groupings of \acp{API} each with their own initialization and finalization functions.
+If a process initializes as either a \emph{server} or a \emph{tool} that process may also access all of the \emph{client} \acp{API}.
+
+A process operating as a \refterm{client} is started directly or indirectly (for example, by an intermediate script) by the \ac{RM} and is connected to the \ac{PMIx} server instance within that \ac{RM} when the client calls the client \ac{PMIx} initialization routine.
+A process operating as a \declareterm{server} is responsible for starting client processes and coordinating with other server and tool processes in the same \ac{PMIx} universe.
+Often processes operating as a \emph{server} are part of the \acf{RM} infrastructure.
+A process operating as a \declareterm{tool} is started independently (e.g., via fork/exec) or by the \ac{RM} and will connect to a \ac{PMIx} \emph{server} to interact with the processes in the \ac{PMIx} universe.
+An example of a \emph{tool} process is a parallel debugger that will connect to the server to assist with attaching to a set of client processes.
+
+\ac{PMIx} serves as a conduit between processes acting in these three different roles.
+As such, an \ac{API} is often described by how it interacts with processes operating in other roles in the \ac{PMIx} universe.
+
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