tk_scsiformat is a Tcl/Tk user interface to scsiformat(8).
You should read that manual page for the general operation of scsiformat. All common X11 and Tcl/Tk options apply.
By the nature of graphical user interfaces, there should not be much to explain here. All entry fields refer directly to scsiformat(8) options.
In addition to those, you are able to save your selections to be used as defaults for this device next time in a file /usr/lib/scsi/scsiformat.defs.* by pressing the <Save Defaults> button. This is intended and very handy for formatting of removable medias. Note, however, that tk_scsiformat is not as clever as scsiformat(8) (or even tries to be) to find if multiple ways to refer to the same scsi device (disk device, generic device).
Furthermore, you can select (or enter freely) a command to automatically make a file system on the newly formatted disk.
While formatting a nice completion bar is drawn. Alas, I could not really implement querying progress from a target device yet.
The /usr/lib/scsi/tworands binary is used to calculate two random integers which is not supported by plain Tcl/Tk.
Old status files in /tmp will confuse tk_scsiformat. However, they are removed after 48 hours.
Restrictions of the SCSI_IOCTL_SEND_COMMAND ioctl(2) call for the sd(4) device make it impossible to issue a FORMAT_UNIT command with more than 4096 bytes of arguments. This could be avoided by using the proper generic scsi device /dev/sg* instead, at least where the kernel is compiled to support it. Most of the time this is not needed though and thus I'm myself to lazy to do it.