|From the filesystem guide.|
The hierarchy is divided into the shareable and unshareable, static and variable.
Static files cannot be manipulated unless the administrator explicitly decides to act on them. They may be binaries, programs, and libraries that came as it was; only under your own discretion may you take access and do work on these files (ex. code revision.)
The variable files, however, are meant to change, stuck in a flux as necessity permits. For instance, when some emails arrives on your Thunderbird client, you can label some, trash others, etc.
There is a clear difference between the shareable and the unshareable files. User files, like our word documents and our music and - to an extent - our information, sit in a pool to be shared. The unshareable files are really device-related, for booting, system-wide tasks, device information, the kernel.
But all these files are open source, able to be editted.