It is a text field that was added about 8 years ago, but never actually exposed in the interface. when it says "Character Sheet Interface", it's talking about the Attributes and Abilities Tab. That field (and that tab) both predate actual Character Sheets on Roll20, so the old terminology can be confusing sometimes. As far as what it's for, you can safely ignore it. Since it is accessible by Mod Scripts, you could store additional things there, but it's hard to find a use for it on Abilities. I could see storing a versioned history of Abilities in JSON format there as part of some specialized Ability Management Script, but it's never really come up. If that same field existed on other Roll20 Objects, it would be far more useful.