What is the difference between @username and .@username on twitter?

[FA]

Someone has told me that the .@username is convention used on the older twitter interface and tweets using this convention are public were as with @username the messages are semi-public.

[Lindsay-Lollypop]

@username are replys that only followers who follow both people on twitter can see
.@username is a tweet that the person wants all their followers to see but is still directed at the person
For example if someone wanted to ask someone on twitter what their homework was they would use @username
But if the person wanted to tweet something to their followers that involved the other person they would put .@username