A little tracker dial fitting inside the Caster Compendium to track extra things such as hit dice, ammunition, sorcery points, rounds of concentration - you name it!
This tracker goes from 0 to 29 and can be placed to the sides of the Spell Slot Tracker during use (using magnets underneath the leather) and can be conveniently stored in the spine in between sessions.
Each Caster Compendium has enough room for two Tracker Dials (0-29), although the Tarot and Double versions can hold up to three if you don't use a Tracker Dial (0-999).