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.
There's a fast way to make this decision instead of defaulting to whatever's easiest to open first. Ask three questions: does this require back-and-forth to resolve, does it carry any emotional weight, and does it need a record afterward. The answers point you to the right channel almost every time.
If the message is one-directional, low-stakes, and doesn't need discussion, a status update, a quick fact, a reminder, it's an email or a Slack message, whichever your team defaults to for that kind of thing. If it needs a real back-and-forth to land on an answer, or if it's carrying any emotional weight, a piece of tough feedback, a disagreement, a change that's going to upset someone, it needs a conversation, not a written message, because tone doesn't survive translation into text nearly as well as people think it does.
Telling someone their project deadline moved by a day: Slack or email, done in one line. Telling someone their project is being cancelled and their role on the team is changing as a result: that's a conversation, in person or on a call, full stop, even if it feels slower and more effort than typing it out.
Meetings deserve their own filter, because they're the most expensive channel, not in money, but in everyone's time stacked together. A meeting is the right call when a decision genuinely needs multiple people's input in real time, when there's real disagreement that needs to be worked through live, or when the topic is sensitive enough that written words would strip out the tone that makes it land right. A meeting is the wrong call when it exists only to share information that could have been written down and read in two minutes, or when it's really just one person's decision being dressed up as a group discussion.
Slack sits in an uncomfortable middle ground worth calling out directly: it feels informal, which makes people use it for things that actually need more care than that, 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 out loud to someone's face, that's usually the sign it doesn't belong in a chat message.
One more test that cuts through a lot of hesitation: if you're not sure which channel to use, imagine reading 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.