Regarding the SVG that you uploaded with the black circle - from a design standpoint, what you have is a vector circle with a black fill and no stroke assigned. Think of “stroke” as you just drew something out and it’s a line drawing… and then “fill” would be if you went back and colored it in. The outline you initially drew would be the stroke and the colored-in area is the fill.
Page 8 of the document @Jules linked to talks about how the Glowforge interprets vector shapes by if they have a stroke or fill assigned to them.
For a cut or score, you’ll want to assign just a stroke to the circle and no fill.
I don’t use Inkscape but here is a screenshot of Inkscapes stroke/fill menu, I believe. (fingers crossed it’s right )
Also, here is a copy of the circle that you uploaded. This has just a black stroke and no fill, so the inside of the circle is transparent. You can download that by right-clicking and save as, or clicking the thumbnail to expand it and then using the download button from the larger “lightbox” version that pops up. You should be able to load this into Inkscape and see the difference between the circles.