When GSAS-II is initially started, the data tree will be empty, but one can then import any number of datasets and any number of phases, up to the limits of available memory for your computer. The data and phases that are read in do not need to all be used in fitting. This information can all be saved in a single file that has extension .gpx, known as a GSAS-II project file. GSAS-II will not allow any other extension other than .gpx to be used for reading or writing these files. The scripting extension used by GSAS-II (GSASIIscriptable), also uses extension .gpx, and the two files are completely compatible. The GSAS-II GUI is an important tool for use in debugging GSAS-II scripts as one can use that to see what information has been placed into the file.