Although XSTAR is designed with the goal of maximizing the flexibility of parameter values and assumptions available to the user, there are likely to arise situations in which the standard set of input options are not sufficient. Under these circumstances the user may want to attempt to modify the source code in order to effect a particular set of assumptions, geometry, etc. Problems for which customization is more likely to be necessary include models with additional heating or cooling processes (for example adiabatic expansion cooling or cosmic ray heating) or different assumptions about line escape probability (e.g. Sobolev escape probability in a medium with a velocity gradient). If so, the internal operation of the code must be confronted. This appendix presents a summary of the code operation in order to aid in code customization.