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"Often" and "Rare" describe the likelyhood/success of stealing items.
For example, look above at "X-Potion": When you fight "Wendigo", the majority of the time that you try and 'steal/mug' anything from it,
the "X-Potion" item won't be what you successfully steal from it, because its a rare item on that monster. Where as any time you fight the "Right Fin of Sin",
almost every time you successfully steal an item from it, it will more than likely be one "X-Potion".
The numbers under "Rare" and "Often" is the number of that specific item that you would steal, if successful. Also looking at the "X-Potion", since its
directly above, every time you steal it from "Guado Guardian C" you will receive 2 "X-Potion"s in the successful rare steal.