Checkboxes are a useful tool used to quickly modify an action or stat. A checkbox has two states, checked or unchecked. The state of the checkbox will determine the change to the stat or action referencing it.

Lets take a practical example, a weapons proficiency.

To start, we have a stat called StrengthMod and a stat called DexMod. If a character has proficiency with their weapon, we'll say that they can use their DexMod instead of their StrengthMod.

We'll name a checkbox WeaponsProf. If the box is checked, we want it to add the Dex Modifier. If WeaponsProf is unchecked, we want the Action to add the Strength Modifier.

We would therefore write our Action like so:

!(1d20 + {WeaponsProf ? DexMod : StrengthMod})

What this is actually saying is this: "Is the checkbox WeaponsProf checked? If so, use DexMod. Otherwise, use StrengthMod."