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The list file can be used to read several postprocess files at once. This includes both mesh, results and graphs files. The list file is just a text file, for a description of it's format please look at the Customization Manual under POSTPROCESS DATA FILES -> List file format.
When reading a file with lots of time-steps, there is an option to read only some of them:
By the default GiD reads all the steps of the analysis, with the value 1. If another number is entered, for instance 10, then one step every 10 steps will be read including the 1st step and the last step of the analysis.