As of Infuse 5, libraries and other info is synced via iCloud and Trakt.

iCloud Sync (Pro Only)

Requirements

• Logged into the same Apple ID on all devices
• iCloud Drive enabled on all devices (details here)
• iCloud Sync enabled in Infuse > Settings

What is synced

• Saved networks shares (addresses, username, password)
• Saved Favorites (folders and Library items)
• Up Next list (in-progress and recently added items)
• Manual metadata corrections (corrections for mismatched titles)
• File specific playback settings (selected audio/subtitle track, zoom setting, etc...)
• General settings (iOS device backups only)

Trakt Sync (Free + Pro)

Requirements

• A free Trakt.tv account (sign up here)
• Logged into Trakt on all devices (login via Infuse > Settings)
• Scrobbling enabled

What is synced

• Watched history 
• Playback position for in-progress items
• Ratings and comments

Note: Sending data to your Trakt account (scrobbling) is available in all versions, but syncing data from your account to other devices (2-way sync) is only available in Infuse Pro.