So I’ve added the extended-tweet tag as suggested, good idea.
If you include a URL in the text body of your Tweet then yes, it will be inside the 140.
The area that we’ve specified as a suffix would be included if you either use the forthcoming attachment_url parameter that we hint at in the documentation (this is intended for use with e.g. direct message deep links), or if a Tweet has attachments such as images or videos uploaded via the API. It will also include poll cards created via the official Twitter client apps.
In the example you’ve posted, which is based on our docs, you have a pic.twitter link - this is intended to show that an image has been attached via the API.
The other thing to note is that the @mentions are in the prefix here because the assumption that this is a Tweet in reply to another, and was created via the server lookup for metadata replies.
So in essence posting 140 characters of text without using any of the new parameters will continue to count as 140 characters. If an app “opts-in” to the new behaviours then there will be additional space within the 140 that would otherwise have been used by URLs and @mentions.
