Whenever a model is created in the astra.tcl program it is output in a variety of files in order to make it available for the main ASTRA program. The ASTRA program then loads in the model to be run from these files and on the basis of these files will access other files that contain data on the object being modelled. Each file is described in turn below. The number in brackets after the title in each case refers to the unit number assigned to that file in the astra.f source code (if relevant - a `n/a' generally implies it is only used in the astra.tcl program).