This cache is created and maintained by the symfony system itself. There is never a reason for a developer to directly modify it.
The developer does however often clear the cache using the cache:clear? task; especially after modifying the class hierarchy in the project. This commonly occurs when executing a propel:build-model? or doctrine:build-model? or a task that aggregates one of these with other tasks.
This directory is all derived from the actual contents of the project. It is not desirable to maintain its contents in a version control apparatus.
Actions called with the HTML POST method or those with GET parameters are not cached.
The cache will contain directories for each of the applications within the project that have been run since the cache was cleared, as well as many ancillary files which are not specifically documented and may be changed in the future.
listing from Jobeet:
> ls -l cache drwxrwxrwx 4 user staff 136 Jan 7 14:59 backend drwxrwxrwx 6 user staff 204 Dec 12 15:35 frontend -rw-r--r-- 1 user staff 86962 Jan 15 13:48 project_autoload.cache drwxrwxrwx 4 user staff 136 Jan 12 13:40 tmp