For details, see the /etc/default/init section of init(1M).
The CMASK variable referred to in the file is not documented in the man page. CMASK is the umask that init uses and that every process inherits from the init process. If not set, init uses the default umask it obtains from the kernel. The init process always attempt to apply a umask of 022 before creating any files, regardless of the setting of CMASK. All values in the file are placed in the environment of the shell that init invokes in response to a single user boot request. The init process also passes these values to any commands that it starts or restarts from the /etc/inittab file.