The model spectrum, f(E), is calculated within XSPEC using the energy ranges defined by the response file :
and is in units of photons/cm 
/s. XSPEC allows the construction of
composite models consisting of additive components (e.g., power-laws,
blackbodys, and so forth), and multiplicative components, which modify
additive components by an energy-dependent factor (e.g., photoelectric
absorption, edges, ...). Models can be defined in algebraic notation.
For example, the following expression:
defines an absorbed blackbody (phabs (bbody)) added to a power-law (power). The result then is modified by another absorption component (phabs).
For a more detailed explanation of models, see Chapter 6.