[galaxy-user] How to use GATK Unified Genotyper with "A list of genomic intervals over which to operate" option

Hi, I would like to run "Unified Genotyper" on a region of a BAM file, I see the advance option "A list of genomic intervals over which to operate" exist and seems to be what I need. The problem is I only get a drop-down menu with the single option "Selection is optional", which I don't understand. Thanks, Carlos