When iTunes asks, "Do you want to erase the contents of this iPhone and sync to this iTunes library?" does it actually erase all of the contents of your phone?

Hi Hanna,

Thanks for participating in the Apple Support Communities.

It looks like you're wondering what will be erased from your iPhone when you sync with a new iTunes library. According to Move content from iTunes on your computer to your iPhone, iPad, or iPod touch:

You might see a message that your device is synced with another library. This means that you've used a different computer to sync your device in the past, and iTunes can't sync without first erasing and replacing all iTunes content on your device.

iTunes content may be any of the following content mentioned in Sync your iPhone, iPad, or iPod touch with iTunes using USB that was synced from the previous iTunes library:

In addition to Music, Movies, and TV shows your tabs might include:
Info: Sync your contacts and calendars.
Apps: Manage your apps and file sharing.
Podcasts: Sync your podcasts.
iTunes U: Sync your iTunes U courses.
Books: Sync your books and and PDF documents.
Audiobooks: Sync your audiobooks.
Tones: Sync your ring(tones) and other alert tones.
Photos: Sync your photos. Learn how to sync photos with iTunes. Turn on syncing.

If you've only synced music from the iTunes library, then only music should be erased from your iPhone.

Best Regards.