Restart files provide a snapshot of the simulation during an analysis. Restart files are used to safeguard against system failure, to pause long analyses, and to extend analyses.
When a system failure occurs and recovery data is available, the analysis will continue from the point at which the last set of restart files were written. To save disk space, the solver will only retain the most recent set of restart files, it will overwrite older restart files when writing new recovery data.
You can pause or break up long analyses by specifying restart steps. When you next launch the analysis, it will continue from the last valid set of restart files.
Restart files are also used to extend an analysis sequence. For example, when you extend a Fill analysis with saved restart files to a Fill+Pack analysis, only the Pack analysis needs to be run, providing you have not changed settings that invalidate the Fill analysis results.