Here's what other sites are saying:
The G3 Onyx Bellowed Boot Shim is a required accessory when using the Onyx with Scarpa's F-series AT boots or other bellowed boots. Does not hinder ski flex and works with G3 Onyx crampons.
Required when using the ONYX or RUBY binding with Scarpa's F-series AT boots or other bellowed boots, G3's ONYX / RUBY Bellowed Boot Shim is easy to install and remove. They do not hinder ski flex and they work well with ONYX crampons as well.
- Mountain Gear