The message ID is an internally-generated ID that uniquely identifies the message.
The Message ID is a number ranging between 100000 and 999999. The chapter is divided into ranges of Message IDs. Within each section, the messages are ordered by Message ID. The best way to find a particular message is by searching on the Message ID.
Throughout the messages, you will see printf(1) formatting characters such as %s or %d. These characters will be replaced with a string or a decimal number in the displayed error message.