I see one mention of scratchpad in the roadmap but I have little idea what it's supposed to be, what sort of capabilities you have planned for it.
Topic on Talk:Structured Discussions
If memory serves me right, it's supposed to be a canvas of free-form wikitext that appears at the top of topcis and/or boards, so that it can held any kind of content that isn't fit for Flow comments.
It was not very detailed because it's not clear yet what its use cases would be.
If I remember well, it was also supposed to be an area for collaborative editing, as opposed to individual posts/replies that can be edited only by their creator. However, this current restriction is just a setting and may still evolve in which case the notion of scratchpad may become less relevant from that respect.
At one stage it seemed that Flow might not support advanced markup such as mathematics and so a scratchpad would be required for any form of collaboration: see Thread:Talk:Flow Portal/Maths/reply (10) for example.
By the way, for added comedy value, see Thread:Talk:Flow Portal/Maths/reply (13) a short way down the same thread.