These jigs are for aligning extrusions when building V1.8. This allows you to precisely align the extrusions with a minimal amount of work.
Pretty much anything works. A good starting point is 2 perimeters and 10-15% infill.
Loosely assemble the extrustion blind joint and then place the entire blind joint onto the jig. While on an extremely flat surface, press down firmly on both horizontal extrusions and tighten the blind screws.