You know what?! That's a great question. I'll try the Promote next time instead of Boost.
I also found this:
"Promoted posts are shuffled across the feeds to all users in mobile app, desktop app and website. There is no limit or restrictions, any post can be promoted to get more exposure and engagement. We hold right to remove posts from promotion list if it violates certain internet etiquette."
You can read more here: https://ecency.com/faq#how-promotion-work