When to use it
Use it when two requests compete for the same time and you can't do both. Name who needs what and when, then ask which comes first. Priorities are your manager's call, and a clear either/or question makes it quick to decide.
Example
You: “Marketing and Sales both need me Thursday. Which should come first?”
Manager: “Sales. I'll let Marketing know it's Friday.”
In an email or Slack message
In Slack: “Conflict for Thursday: Marketing wants the banner, Sales wants the deck. I can do one Thursday, the other Friday. Which comes first?”