In order to minimize the computer costs, all programs in the Aqwa suite have the optional facility of stopping and starting at various stages of the analysis. These stages are referred to in the documentation as Restart Stages.

This facility enables the user to check the results of the preliminary stages and detect any misinterpretation of the data input instructions or errors before the more expensive stages of the analysis are performed. When the user is satisfied that the results are correct, the data categories relating to the previous stages are removed for the remaining part of the analysis, as they are stored in a backing file.

When errors are detected, the user instructs the program to start the analysis again, at any of the preceding stages, before the error occurred. This process is referred to in the documentation as a restart. If the previous stage was correctly performed, the user instructs the program to execute the next stage, i.e. simply continue the analysis.

Use of the RESTART process thus implies that information is available on a backing file from a previous program run and hence is also used to transfer information from one program to another program in the Aqwa suite.

The data input in Data Categories 1 to 18 fall into three main categories which also correspond to Restart Stages 1, 2 and 4. These categories are described in Data Categories 1 to 5 - Geometric Definition and Static Environment, Data Categories 6 to 8 - The Radiation/Diffraction Analysis Parameters , and Data Categories 9 to 18 - Definition of Analysis-Dependent Parameters in the following text. It is worth noting that Stages 3 and 5 are those which perform specific analysis tasks.