If you draw a vertical line from top to bottom, then parallel "left" goes to the right. If you draw the line bottom to top, then parallel "left" indeed goes to the left. Similarly with a horizontal line – on a line drawn left-to-right, "left" goes up; on a line drawn right-to-left, "left" goes down. Polylines (as a polygonal line) follow the "in" and "out" pretty well – but once you break the polyline into individual lines it mostly seems to go in the opposite direction to that which you expect!
I usually work with both parallels turned on and simply delete the one I don't want. Some means of telling the program which way the parallel should go would be very useful (clicking on the line and dragging in the desired direction would be good); then perhaps clicking on the line with option held down would go in both directions, as for drawing parallel double lines etc.
What RealCADD does really well is draw parallels to polylines and other complex shapes and constructions – I wouldn't want to lose this!!!
