int WINAPI Compare(
  HANDLE hPlugin,
  const struct PluginPanelItem *Item1,
  const struct PluginPanelItem *Item2,
  unsigned int Mode
);

This function returns an int value that is:

-1 if Item1 < Item2
 0 if Item1 == Item2
 1 if Item1 > Item2
-2 if the default FAR compare function should be used for this sort mode.

The standard RTL qsort function that implements and unstable sorting algorithm is used by FAR for sorting needs. In other words if array elements are equal to the compare function then on the panels they will be shown in random order, that changes upon each redrawing of the panel.