I am unclear as to where exactly one is supposed to downvote reasonably.
I find that most people downvote newcomers' questions, since they lack context and formatting. But I personally feel they must be given a chance to edit their questions to suit the site's need. So, I avoid downvoting there.
People who have an appreciable reputation already know how to ask things here, so they don't generally make errors in their questions.
I understand answers which do not give valuable content relevant to the question or to the OP's understanding must be downvoted.
Leaving these cases, where must one downvote to show that the content is not useful to the site?