Mutable vs. Immutable Areas of the Repository /apps and /libs are considered immutable areas of AEM as they cannot be changed (create, update, delete) after AEM starts (i.e. at runtime). Any attempt to change an immutable area at runtime will fail.
Everything else in the repository, /content, /conf, /var, /etc, /oak:index, /system, /tmp, etc. are all mutable areas, meaning they can be changed at runtime.