You can specify up to 10 spars or reference lines for the plot. These can be true rectangular spars, or simply vertical lines drawn across the airfoil for your reference (for instance, to mark where your trailing edge stock begins).
The spar position is required to draw anything, and can be specified as either a percent of the chord or an absolute distance in the chosen units. If the value is positive, it's measured from the leading edge; if negative, it's measured from the trailing edge.
For percent values you need to add a percent sign (%) to the value you enter, so for instance to position a spar 33% of the way back from the leading edge, type "33%" in the position box.
The optional spar width is specified in the current units. Zero width is OK (and leaving the width box blank is interpreted as zero width): a zero width "spar" is just a single vertical line. Negative widths aren't allowed. If you specify a width but no depth, you will get two vertical lines, spaced "width" apart, centered around the position you specified.
The optional spar depth is also an absolute value in the current units, but it can be negative: negative values signify that the spar is on the bottom of the airfoil, and the depth is measured from the bottom surface. A zero value (or leaving the field blank) tells the program to draw the spar across the full thickness of the airfoil, while a non-zero value tells the program to draw the spar only to a certain depth. If you need two spars directly above and below each other, add two separate spars with the same position and width, one with a positive depth and the other with a negative depth.
The checkbox next to each spar signifies whether to rotate the spar to make its surface flush with the surface of the airfoil. If it's checked, the spar is rotated to match the foil surface; if not, the spar sides are always vertical. Note that this checkbox only has meaning if you have specified both a width and a depth for the spar (in other words, a real spar, not a reference line).
If you specify a spar that is outside the bounds of the airfoil, it won't be drawn. Overlapping spars that have both width and depth are not allowed.