Presumably files exported in sysfs. The very nature of sysfs makes keeping it stable an awkward task. As the first few lines in Documentation/sysfs-rules.txt say:
> The kernel-exported sysfs exports internal kernel implementation details
> and depends on internal kernel structures and layout. It is agreed upon
> by the kernel developers that the Linux kernel does not provide a stable
> internal API. Therefore, there are aspects of the sysfs interface that
> may not be stable across kernel releases.