Among them, it will no longer count usernames sent in reply tweets and media attachments in 140 character limit, giving users the full space to express themselves. The changes, to be rolled out gradually over the coming months.
For instance, when a Twitter user wanted to reply to another tweet and have all of his followers see that reply, he had to know to put a dot in front of the "@" symbol preceding the other user's handle. Without that workaround, the tweet would be seen only by people who follow both accounts.
Source: The Wall Street Journal, May 25, 2016