That button is for when you want to quote multiple posts. If you click on the Multi-Quote button you will notice that it turns red. Then you can click on the Multi-Quote button for another post and that will also turn red. If you then click on ‘Quote’, then all of the posts that you have selected will appear in your reply.
It’s a little confusing because you might expect it to quote multiple parts of an individual post. What it actually does is quotes multiple *whole* posts.
Separately, if you want to quote just part of a single post, use the ‘Quote’ button and delete the bits you don’t need.
Yes — thank you for the explainer.