This flag enables a split (or dual-pane) mode, allowing two sets of columns to be scrolled horizontally independently. This is typically useful when you have a lot of columns (so horizontal scrolling is needed) but you want one column, or perhaps a few columns, to always be visible. By putting those columns in the left pane, they will be immune to the horizontal scrolling taking place in the right pane. Two fields in the xtrctl structure, XTR.LEFTPANECOLS and XTR.LEFTPANEWIDTH, are used to control column size and spacing; see those topics for related inforamtion.
In most respects, the dual pane mode should act just like the regular, single-pane mode. Both panes remain vertically synchronized at all times. However, in editable mode, there is currently an assumption that you would probably confine the editable cells to the right pane, and thus the editable cell navigation operates on this principle, although you could still select an editable cell in the left pane with the mouse.
Two sample programs are available: XTRA2S is a variation of XTRA2, with the zip column in the left pane by itself. XTRA5 has been upgraded to offer an option of how many columns, if any, to put in the left pane. (If none, it works as before in single-pane mode.)