If you can see this check that
If you were looking for a top level directory in which to find
system files which may change at runtime, which directory would
you consider first?
This directory holds kernel state information, such as what disk drives
are installed and what each of the running processes in the system is up to.
/usr holds readonly system files which are not usually needed till after the
initial boot process. Traditionally this directory could be shared amongst
multiple machines using a readonly NFS mount. Many big packages are installed
here, like word processors.
/var is indeed designed for system writing, and includes the spool directory,
the log directory, and email.
This stores readonly system files. These files are not written to by the system
as part of its normal operation, but files may be changed by the system
administrator as part of configuration or user changes.
This is used to write temporary data which may last until the machine
is rebooted. Often this is referred to as scratchspace. It should never
be used for important or private information.