I'm trying to find out what does "iTunes Accounts" traffic stands for on iOS, since it has gobbled 850MB on cellular data in 13 days. I have iTunes Match on but have disabled the Music app to access data on cellular networks. I'm on iOS 8.3 since today, data was consumed on 8.2.
Probably iTunes & App Store background downloads.
You can prevent these, so it only updates when you're connected to wifi, in
Settings > iTunes & App Store… by disabling Cellular Data
[I also prevent all automatic downloads, so mine only updates from iTunes itself, on my Mac]
Seems like it might be iCloud documents syncing Settings -> iCloud -> iCloud Drive (scroll to the bottom to turn off cellular data).
Also seems like iTunes music "new music mix" might come in under this.
Wow, my phone (iOS 10 now) shows this as 129MB in the last month since reset, with all the above turned off
I don't even play Music on my phone, only listen to Podcasts (which has a separate total of 74MB).
Sorry I don't have an answer but thought this might be a useful data point confirming that a significant amount can be run up without the data being enabled tor iTunes & App Store