Consider the following document:

Wouldn't it be nice to just express your feelings right away?

Substance offers inline annotations (markers) as well as comments to allow exactly that.

Eventually consistent

Most things in life are not perfect. That's also true for every document ever written or to be written in the future. Substance supports a workflow that assumes documents will be consistent eventually. We claim that it's way easier to reach quality if you publish early and provide an easy way for collaboration. Substance helps with implementing a peer-review process, with every reader being a peer.