Integrations with Apple's own services — Game Center leaderboards and invites, iCloud sharing, contact-based invites — can't be used to blast unsolicited invitations or notifications to other users at scale.
Common with social or gaming apps whose "invite friends" flow auto-sends messages to a user's entire contact or Game Center friends list without individual confirmation.
"Your app sends unsolicited messages or invitations to users via Apple services, which is considered spam under guideline 4.5.3."