This dialog box is only displayed when you choose one of the "Modify | Block" commands and no text has been highlighted. It lets you determine the size of the block that you want to process. The line that will contain the top left corner of the block is defined by the cursor position. Measurements in this dialog box represent character units.
Note that the block commands are not designed to operate on word-wrapped lines. Currently, NoteTab treats tabs as a single character so tab size is not taken into account - this feature may be improved in a future version of the program.