Briefly: consider the Paper Trail rule proposed in the article above. There’s often messages that don’t quite fit the terms they’ve suggested that would be “paper trail” messages. Conversely, there’s messages that might contain the term “invoice” or whatever that are should actually be screened.
So, I’ve got an Info group. I add contacts into that group, and their status in the group filters the messages.
The nice thing about Fastmail, of course, is that you can do both of these approaches depending on what you’re trying to achieve.