XSPEC places each data set into a data group. Each data group has its own set of parameters for the defined model. These parameters can be either independent from data group to data group, or they can be linked across data groups using the standard XSPEC syntax. At present, each data group must have the same model, but some components can have normalizations set to zero. If no data group is specified, then the default is to place all data sets in the same data group. The model must be redefined when the number of data groups in use is changed.