It probably depends on the devices and the Android versions.
On Replicant 6.0, it contains at least the application internal data: When various applications are installed, they have access to some storage where they can read and write data that is only visible to them and not to other applications. This is part of the Android security model that doesn't trust applications even if they are fully free software.
In addition, on some devices it also contains the users data. This location and external microSD cards is where users typically their files like Music, photos, etc. This data is made accessible to the users in various location through a mechanism called fuse. In Replicant 6, this data is stored in the media directory in the data partition.
Summarry:| Mount points | Content |
| /data/ | Applications (internal) data |
| /sdcard /storage/<UUID> |
User accessible storage to store music, photos, etc |
Here's where the sdcard user accessible storage comes from:
/sdcard -> /storage/self/primary [1]
/storage/self/primary -> /mnt/user/0/primary [2]
/mnt/user/0/primary -> /storage/emulated/0 [3]
1
root@i9300:/ # readlink /sdcard /mnt/user/0/primary
2
root@i9300:/ # readlink /storage/self/primary /mnt/user/0/primary
3
root@i9300:/ # readlink /mnt/user/0/primary /storage/emulated/0
4
root@i9300:/ # mount | grep "/storage/emulated" /dev/fuse on /storage/emulated type fuse (rw,nosuid,nodev,noexec,noatime,user_id=1023,group_id=1023,default_permissions,allow_other)TODO: