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Iris was devoted to provide functionality in a specific scientific domain, namely the analysis of broad-band SEDs. This requirement clearly defines the semantic scope of the Iris extensible framework, and provides a clear abstraction layer to both users and developers, inside and outside of the development team.  In this paper we present Iris, its design, and how users and developers can plug their code in it. In section \ref{sec:overview} we briefly explore the landscape of applications and analysis tools in which Iris is inserted. An overview of the Iris general architecture (the Iris \emph{stack}) is illustrated in section \ref{sec:stack}. We introduce the Iris features in section \ref{sec:builtin}, and explore how astronomers can plug their models or templates in as Python functions. functions in section \ref{sec:usermodels}.  We then move to the details of the Iris extensible framework design (section \ref{sec:architecture}) and to a somewhat detailed description of the more advanced Iris functionalities (section \ref{sec:components}). Finally, we provide more details about how to extend Iris via Plug-ins (section \ref{sec:plugins}).