Here's the exact text from the nominator onboarding training that I do each month:

Topics matter

Ideally, every nominated story has at least one of our Explore page topics represented – this is especially important if the story author is new to Medium or has very few followers.

If you're a publication editor, we suggest coaching your authors to ensure that their posts are well-tagged.

Missing or obscure tags won’t block a story from being Boosted, but thoughtful topic tags will support a Boosted story performing well.