There is no formal definition of how to use it, but the current usage seems to be to use Upvotes to indicate that you think a post is ready for main and to use Downvotes to indicate that you think a post is just not a good fit for the site and is basically unsalvagable. Personally I tend to not use downvotes, but that's how I feel they are being used. Most of the time you won't see any voting on posts in the Sandbox though. Voting in either direction is relatively rare overall and you shouldn't think too much about the vote count for drafts in the Sandbox.
Personally I recommend to use votes in the following way:
- use upvotes if you think a question is ready for Main in its current form
- use downvotes if you feel that a question is completely unsalvageable, even after multiple edit cycles by the OP - this should be used very rarely and be explained to the OP
It's a Sandbox intended to be used for drafts that you know will be difficult on Main or for people having difficulty phrasing their questions in such a way that they will work on Main - it's normal that drafts there don't look good on the first few revisions, so please be patient. We don't want to discourage people who want to spend some time on working out the details of a question.
But if you feel that a post is ready or that a type of question will never work voting accordingly can be helpful - ideally you would comment so that the OP knows that they are probably ready or that they should try to rethink the whole premise of their draft. While we don't want to discourage people using the Sandbox we also don't want to waste their time.