An inguinal hernia must begin at the deep ring or the posterior wall defect. It must pass into the inguinal canal and then it must pass anterior to the inguinal ligament as it passes through the superficial ring.
A spigelian hernia must pass between the lateral edge of therectus abdominus muscle and the lateral abdominal wall muscles (external oblique, internal oblique, transversus abdomenus).
A spigelian hernia may enter the sub-cutaneous space or split between the lateral abdominal wall muscles.
A spigelian hernia never enters the inguinal canal and never passed through the deep or superficial ring.
So anatomically they are in quite different spaces.
I find differentiation of femoral Vs inguinal hernias to be a challenge sometimes and the key difference is that femoral hernias pass posterior to the inguinal ligament and inguinal hernias pass anterior to the inguinal ligament.