Reader Aaron Landers is confused by an element of iTunes syncing. He writes:
I have some songs on my iPhone that I’ve downloaded via iTunes Match. On this same iPhone I want to install some ringtones that I made in GarageBand via iTunes. But when I choose to sync ringtones, iTunes threatens to erase all the music on my phone. I’m afraid to try so can you tell me what will really happen?
I’d be happy to. This warning tells me that you last synced your iPhone with a Mac different from the one you’re attempting to now sync ringtones with. And because you are, iTunes is telling you that when you choose to sync from the new Mac, content copied from the old Mac will disappear. And that’s correct. Any ringtones or music copied from the old iTunes library will be vaporized when you sync the phone with the Mac you’re currently jacked into.

This warning may not be as dire as it seems.
But note the words “from other iTunes libraries” in the dialog box. Having noted them, take them literally. The tracks you’ve added via iTunes Match aren’t from another iTunes library. They’re from the cloud and are therefore exempt from iTunes’ vaporizing ways. Even if you turn off iTunes Match on your phone and then sync music via iTunes, the iTunes Match music you’ve already downloaded will remain on your phone.
Confusing? Absolutely, and one more argument for syncing everything from the cloud so that you needn’t worry about which hunks of data belonged to a particular iTunes library. The difficulty here is that ringtones aren’t synced with iTunes Match. And while purchased ringtones can be redownloded via iCloud, custom ringtones can’t.