CORS - When to return `Access-Control-Expose-Headers`

Question

Should the Access-Control-Expose-Headers header field be returned along with an entity in response to an 'actual' request?

Or should it only be returned in response to a CORS preflight request?

Or both?

Answer 1

The flowchart is such a good resource I wanted to repost it here in my own answer.

Image from: http://www.html5rocks.com/en/tutorials/cors/#toc-cors-server-flowchart