for the record I dug this out of the [not very search-friendly] xara help:

Note: A zero width line is a special case, and should not normally be used. This draws a line one device-pixel wide. The actual size will therefore depend on the device the drawing is displayed on. On a monitor screen one pixel is clearly visible and is never shown anti-aliased. On devices such as printers and particularly on image setters, which have a much higher resolution, the line is almost invisible (zero width lines in imported files are converted to 0.25pt).