What is this 'r=xxx' suffix on my share URL?

Twitter shares that include a URL (Either custom or context aware) will have the referrer parameter '?r=xxxx' added to them. This parameter is used to track twitter referrals from your campaign. This feature cannot be disabled.

Does this add to the Tweet character limit?

This does not decrease the number of characters available in a tweet. No matter how long or short the link Twitter will count any URL you place into a Tweet as 22 characters, or 23 for HTTPS links.

Further details

If you include a URL into a Tweet, then you only ever have 118/117 characters to use, this also has a cumulative effect, if you place two URLs in a Tweet, it will count for 2 x 22/23, so 44/46 characters, further reducing the number of characters you can use. This means that the referrer parameters adds nothing to the character count of a tweet.

The platform character count for twitter sharing will show the correct number of remaining characters.

Appearance

The appearance of your URL within the tweet will be as you typed it or as the context aware campaign share shows it (ngx.me/shortcode&r=xxx). If the URL is more than 22 characters, it will be cut the length to 22 characters and replace the last 3 characters with '...'.For example:

The actual link will be in the format t.co/xxxx, which you can see in the status bar of your browser when you hover over it.

Sources

https://dev.twitter.com/rest/reference/get/help/configuration
https://dev.twitter.com/overview/t.co