The most important addition to the science capabilities added in XSPEC12 is the ability to fit multiple sources simultaneously with models of different functional form. For coded mask apertures, a different effective response arises from different parts of the field of view. XSPEC12, unlike XSPEC11, can assign separate model components to these different responses.
This has use where source confusion is present and also when the data are dominated by background flux. This capability was largely motivated by support for the SPI instrument on the INTEGRAL spacecraft, but is likely to have wider applicability to gamma ray astronomy conducted through coded mask instruments if the field of view is wide and/or the instrumental background is