What Does Bd Mean By "message Number"?

When we get the messages about BD not being able to delete Inbox messages because they are archived, in many cases it just puts out a message number without any message text, so it is unclear what that "message number" means. For instance, is message # 30 in fact the 30th group of records in the Inbox starting with "From - " at the beginning of a record? How does it assign "message numbers"? I am using Thunderbird as my email client.


Thanks in advance,