This is a count of write-to-mailbox requests performed system-wide since the system was booted, mailboxes being VMS's inter-process communication channels. An administrator monitors this counter to gauge how much inter-process communication activity is occurring on the system. For example, an unusually high vmsIoMailboxWrite count could indicate a set of cooperating processes exchanging messages at a much higher rate than normal, worth investigating if it correlates with a performance issue.