Hi Lars,
The quote feature (ie the "quote" button) enables you to quote the entire message which you want to reply to. It's not possible to select the text to quot, before you click on the button and are transported to the "reply" section. When there you can edit the amount of quoted text to suit your needs.
The "multi-quote" button tags the messages you want to reply to in any given thread. After "tagging" a few posts you hit the quote button on any of the posts and you'll be transported to the "reply" area where all the tagged posts should appear in quotation, one below another, for easier quotation/reply to specific quotes.