On a serious note: interprocessor communications often use "mailboxes". You have an inbox and an outbox. You send messages to other processor(s) by putting a message in the outbox. You receive messages in your inbox.
When powering on, the III displays diagnostic messages during the first phase of the boot routine.
This includes "checking mailbox":
"Going all the way back to the original Axe-Fx I used a concept called a "Mailbox" to communicate between processors. Each processor has an inbox and an outbox. I'm self-taught in programming and computer science so it was just a way I came up with to send data back and forth. Turns out that mailboxes are actually a technique used in interprocessor communications. In this specific case the processor is checking the mailbox from the front panel to see if any buttons were pressed at power-on."