Most people pick the channel out of habit, not judgment. That's how simple updates turn into meetings and hard conversations turn into a Slack thread nobody wanted to be in.
Ask three questions before you default to whatever's easiest to open. Does this need back-and-forth to resolve? Does it carry any emotional weight? Does it need a record afterward? The answers point you to the right channel almost every time.
One-directional and low-stakes goes in writing, a status update, a quick fact, a reminder, as an email or Slack, whichever your team defaults to. Needs real back-and-forth, or carries weight, tough feedback, a disagreement, a change that's going to upset someone, becomes a conversation instead. Tone doesn't survive translation into text nearly as well as people think.
Telling someone their deadline moved by a day: Slack or email, done in one line. Telling someone their project's cancelled and their role is changing as a result: that's a conversation, in person or on a call, even if it feels slower.
They're the most expensive channel, not in money, in everyone's time stacked together. Run this test before it goes on anyone's calendar: is this for making a decision together, working through something that needs real-time back-and-forth, or resolving disagreement that hasn't moved over email? If the honest answer is "no, I just need to tell people something," it's not a meeting. It's a message.
If nobody in the room needs to say anything back, it wasn't a meeting. It was an announcement wearing a calendar invite.
A recurring status update where each person reports what they did that week, no discussion, no decisions, just information moving one direction.
A planning session where three people with different constraints negotiate a shared timeline live, because the tradeoffs only surface through back-and-forth.
If you're the one calling the meeting and the honest test says no, say so before it hits anyone's calendar. The line works whether you're the organizer catching yourself or the invitee catching someone else.
"Would it work to send this as an update instead, so we can save the time for things that need discussion?"
Most people appreciate being asked, even if they hadn't thought to ask themselves.
Slack sits in an uncomfortable middle ground worth naming directly. It feels informal, which makes people use it for things that need more care, a piece of feedback, a "can we talk," a decision that affects someone's role. If what you're about to type would be a hard sentence to say to someone's face, that's the sign it doesn't belong in a chat message.
One more test for when you're still not sure: read the message back with no tone of voice attached, cold, on a bad day. If it could land wrong, it needs a real conversation instead.