I had created a full (boot+recovery+system+data) backup to an external SD card in TWRP 3.0.2-0, with compression enabled. Flashed another firmware, played a little then desired to restore the previous one. Booted TWRP, selected Restore, pointed to the backup. TWRP reported a successful restoration. After a reboot, all system setting and applications with their "internal" data (/data/data) had been restored. But user data (/data/media/0, bound to /sdcard) were not restored.
I booted TWRP again and performed a separate data restoration. Successful completion report but no user data in /sdcard. /sdcard looks like wiped, containing just basic Android directories.
I unpacked data.ext4.win000 and data.ext4.win001 to a single .tar file using 7-Zip (under Windows) and looked on its contents. All user data file paths are there but they begin from /storage/emulated/0. Entering TWRP's File Manager, I see there is no /storage mount point at all.
Since .tar file contains uncompressed data, I tried to find some text fragments of user data stored in /sdcard using a hex editor. I found no such fragments.
Therefore, /sdcard contents (from /data/media/0) is not stored in the backup. So all /sdcard contents will be lost if not stored separately elsewhere.
Definitely, it is not a "full Nandroid backup", only a "system app+data+settings" backup.
So why /sdcard file paths are stored as /storage/emulated/0 if TWRP does not create /storage mount point?
And why /sdcard file contents are not stored in the backup?