After it starts, it updates the disk usage of the partition it was started with. The usage is shown on the gauge, and the partition is displayed on the footer. Since xdf updates every three minutes to avoid overusing computer resources, a sudden jump in disk usage will not show up immediately. It can be used to compare histograms between phase sets and between different crystals. The histogram for a correctly phased protein with a given percent solvent and resolution range is characteristic.
Any deviations are due to phase errors.
By comparing your phase set with those of other well-refined proteins in the same resolution range, you can guess the amount of phase error. The histograms can be saved and loaded in the File window. File Pop-up Window has several options, as described below. Directory tells where to find the data files. List lists the files matching the displayed filename filter.