hi Seth Mak,
1) What is the difference between "Draft" & "Revision"
Actually both is almost similar, let me try explain as simple scenario :
User A and User B - Author team
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User A try to write a blog post regarding the 'e.g. Recycle propagate', after completed his post then save as draft first.
So User B come to check User A draft post and see is it everything is correct.
After User B checking it and everything is correct so User B publish the blog post. (so this time system will stored this as Revision 1)
After few day, User A got a new ideas need to add some content in the just now the blog post, so he click edit post and add some new content in the blog post. (this time system will stored as Revision 2)
So from this post history already stored :
1. Draft
2. Revision 1
3. Revision 2
If User B take a look of the blog post, he don't like the new ideas content, so he can load back Revision 1 and publish it.
Also you can do the comparison what changes you did. Check my screenshot in below attachment.
2) It seems that I cannot preview an unpublished "Revision" in the front end. Is it true?
Yes, you can preview your revision post, take a look of my screenshot below.
Hope this help.