Very confusing indeed. :-/

tl,dr: helpful = upvoted by users; useful = marked as useful via the AFTv5 special page by a wiki editor or admin or someone with the sufficient privileges (probably just aft-reader or aft-editor)

So by default when you're privileged enough to "act" on feedback posts, you see a "View reader feedback" link on Talk: pages. Clicking on that takes you to a page like Special:ArticleFeedbackv5/Main_Page?ref=talk&filter=featured -- note the filter=featured part: this ensures that only posts from the "featured" queue are shown to users who follow that link.

Looking at the English i18n file, the confusion only gets worse:
"logentry-articlefeedbackv5-feature": "$1 {{GENDER:$2|marked}} [[$3|feedback post #$4]] as useful on [[$5]]",
The message key contains the word "feature" but the text shown to the user uses the word "useful"; and of course there's a separate message for the helpful case (which, pardon the pun, helpfully uses the same word in both the message key and the user-facing UI text).

Likewise, "unhelpful" feedback is just feedback with downvotes, nothing fancier.