This is the command for entering filters to be passed to extract. The command is
xsel > filter <filter_what>
Each filter command builds on the list set up by the previous ones. For all the filter commands, a file name proceeded by a - indicates that that file should be removed from the list.
Any routine that runs extract as a preliminary step, e.g. sisclean will store this extractor output for another running of the same command. However, entering a new filter invalidates this output, so filter will erase it. However, you will be given a chance to save the file first.
There are three filter_what values (besides quit), TIME, REGION and PHA, which will be treated in their own subheadings.