From what I understand...
If you are a creator and you unlock Ink support (either by reaching 250 subs or getting it unlocked for Inksgiving), you can earn Ink through ad revenue and your readers can donate Ink to you. That Ink can be cashed in for Real Life Money once you reach $25USD.
If you earn Ink as a reader through events and surveys, that Ink cannot be exchanged for money. That Ink can be used to access Premium comics and novels, and to donate to your favorite creators so they can exchange it for cash.