This read-only integer reports the total amount of RAM available to the operating system, which is typically slightly less than total physical system memory and can be further restricted by the 'mem' boot parameter. An admin uses this as the baseline denominator when calculating memory-utilization percentages for the host. For example, an admin noticing this value is much lower than the physically installed RAM would check whether the 'mem' kernel boot parameter is unintentionally limiting usable memory.