I've heard a bit about this before, but I'm still confused about why there need to be different Progressive and Constructive button types. 

I think as far as users are concerned, there are only essentially two kinds of buttons they look for: Yes or No. (e.g. Confirm or Cancel, Save or Discard, and so on). We definitely need a neutral button type for "third options" like Preview or Review changes. But two types representing positive response and moving forward? Seems like we're maybe overcomplicating things. 

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