Mixing models perform transformations on the available spectra. The spectra must be assigned to more than one data group in order have any effect. Each data group is a ``region'' of the observation, and the ``mixing transformation'' allows the model flux from one such ``region'' to influence another region. Thus, these models, unlike all the others, can be two-dimensional in effect. It follows that they differ from all the other models in that the data must be read before the model can be defined. In most cases also, the input data must contain additional information in order to use the model. This additional information can be in the form of the OGIP standard XFLT keys, which allow a set of scalar real values, or in some cases additional files to be read containing spatial information.
XSPEC will return error messages if the data are not loaded, not compatible with the model (i.e. do not contain the required additional information), loaded inconsistently with the use of the model (the division of data into regions is incorrect, or data within a given region fail consistency checks). It will also return error messages if the data are subsequently changed to a set that violates these consistency checks, and additionally remove the model definition.