Recipe Feedback
When inserting Variable Cards into the Crafting Interface, you will either see a green checkmark or a red cross to indicate if the given recipe is valid for the bound machine. You can hover over these symbols to get more information.
This is an early-feedback system in cases you accidentally insert the wrong recipe into an interface, for example if you insert the crafting recipe for a chest into a Crafting Interface that is bound to a furnace.
If you see a red cross, this means that the Variable Card either not exposes a valid recipe, or if the recipe can not be crafted in the bound machine.
A green checkmark indicates that the Variable Card contains a valid recipe, and the machine can (probably) handle the given recipe.