The draw macro takes one argument which specifies which characters you want to use for horizontal and vertical lines as well as intersections and corners. You can also use gui-draw to specify the argument from a radio button.
On the GUI, you can select single lines, double lines, horizontal double line, vertical double line, ASCII characters, or blank spaces.
The help text for the draw function says:
1 Single line top/bottom sides and left/right sides.
2 Double line top/bottom sides and left/right sides.
3 Single line sides and double line top/bottom sides.
4 Double line sides and single line top/bottom sides.
B Draw with blank character. Used to erase drawing.
Any character Any character may be used as the draw character.
The A parameter is not mentioned.
For modes 1 to 4, you have to be using the OEM Fixed Font or another font that supports DOS character graphics, otherwise other characters will be displayed that don't make sense.
I also found that if I'm using the OEM Fixed Font with single lines, it seems to works perfect. It doesn't write intersection symbols on the sides of a simple box when retracing and the intersections work.
However, if I keep pressing Cursor Down on the last line of the document, the line alternates between the vertical line-draw character and a question mark, like follows:
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