Thickness determines which frequencies a panel can actually control.
As a general rule: 3 inches is enough for high-frequency treatment (flutter echo, harshness). 4 inches gives a good all-round balance for most rooms. 5–6 inches is what you need for meaningful low-frequency control.
Corner bass traps are most effective at 5–6 inches, since low-frequency wavelengths are physically long and need real depth to absorb rather than just deflect.
Every edition offers a different depth range — Core runs 3–4", Artisan 3–5", and Signature 3–6" — so your choice of edition and your depth requirement usually point to the same answer.