What does data partition in nandroid backup contain? I've been trying to find information on what's included in the data partition when doing a nandroid backup.

I couldn't quite figure if it includes:


Media files (images, videos, music)
User apps + Configs
System Apps + Configs
Whatever else...


Reason being, I wondered if I could just backup system and boot:


The media files are synced using rsync.
The system apps are updated anyway when flashing new ROM. 
The user apps are backed up using titanium or whatever.


This could save some space of my backups,
since I don't have much to spare -
having no external sd slot and only 16gb of storage...
I think that the best answer is here.  As an Android Expert, I couldn't have said it better myself!


  To my knowledge, TWRP does a nandroid backup. As you can read in the
  tag wikis of the mentioned tags (nandroid, backup), a Nandroid backup
  takes images of your partitions. It's not a backup on the file level,
  but on partition level. So it includes everything stored on those
  partitions. Doing a full Nandroid backup of your system thus includes
  the entire system, one file per partition. There are only very few
  things not contained, such as the radio-firmware.
  
  So to put it as a short direct answer to your question: Yes, a full
  Nandroid backup would include all your apps, app data, messages,
  contacts.
  
  Though it might be complicated to extract them separately (Nandroid
  can't do that, it's always an "all-or-nothing". But e.g. Titanium
  Backup can read data from Nandroid backups. Just in case you'll need
  that :)