When to use it
Use it when someone hasn't replied and you need an answer. Reply in the same thread, restate the specific question, and give a date. That way the person can answer in one line without digging through the earlier message.
Example
You: “Following up on the budget question below. Could you reply by Wednesday?”
Client: “Yes, approved. Sorry for the wait.”
In an email or Slack message
In Slack: “Following up on the budget question above. Could you reply by Wednesday? If it's easier, a thumbs-up works for approval.” Making the reply easy speeds it up.