https://forum.freecadweb.org/viewtopic. ... 25#p362360
... there are basically three types of external geometries that are "projected" in to the sketcher plane (1) normal projection of the external geometries (2) silhouettes (reflect lines) projections of rounded surfaces/faces and (3) intersections of external geometries with the sketcher plane. And if I am not mistaken FreeCAD supports just the first type...
... it is also possible to sort of work around for type 2 and 3, that is to create the needed geometries outside the sketch with for example the PartWB section tool or CurvesWB reflect lines tool and then project (import) those in to the sketch, but ideally sketcher would have support to create the projections of this automatically.
The image below visually represents the three types mentioned above, FreeCAD currently supports only the blue one (type 1), this discussion is about the orange one (type 3), the discussion in the above link is about the pink one (type 2).
Also, about the intersecting type, if we take for example the cube from this example, if we select an intersecting edge of the cube, the result would obviously be just a point, if we select an intersecting face the result would be a line as shown in the example (or a curve, for not flat faces) and if we select the full cube, the result would be a closed profile of lines that represent all the faces of the cube that intersect with the plane...
Here is another topic on some additional limitations of "type 1"
https://forum.freecadweb.org/viewtopic.php?f=19&t=46182 (update: fixed now)
This model is of course just an example, imagine the datum plane to be the sketch and I know that the extracted edges of the cube are not parametric, should have used a different tool but not so important for this example...