A protector compatible ski backpack has a dedicated sleeve or pocket that holds a removable back protector insert, but the insert is sold separately. This lets you add CE-certified spine protection when you want it and remove it when you don't—like stripping weight for a long uphill skin track.
The big advantage is versatility. On resort days or when you're riding technical terrain, snap in the protector for impact coverage. On mellow touring missions where every ounce matters, leave it behind. The trade-off is that buying the insert adds cost, and you'll need to make sure you get the right one for your specific pack model.
This option is ideal if you want the choice to run protection some days but not others, or if you're not sure whether you'll want it long-term. It's also great if you already wear a separate back protector vest and just want the option occasionally.