This dialog opens when you use the Read item in the File menu to read from the current open file and that file has more than one section in it. If you just want to read one section, you can enter the section number and type 'Enter' or press OK, and the dialog will go away after the image is read. However, you can also leave the dialog open so that it is convenient to read images from files. Specifically:
Press Read to read in the section indicated in the text box and leave the dialog open.
Press Next to read in the next section, update the number in the text box, and leave the dialog open.
Press Previous to read in the previous section, update the number in the text box, and leave the dialog open.
Press OK to read in the selected section and close the dialog.
Press Cancel to close the dialog without reading in a section.
Sections are read from the current file, even if you change the current file after opening this dialog. Like many other free-standing dialog boxes, this dialog will be opened at the position where it was when it was last closed.