AutoIt (Macros)

From The Ultimate Programming Reference

Macros

AutoIt has an number of Macros that are special read-only variables used by AutoIt. Macros start with the @ character instead of the usual $ so are easy to tell apart. As with normal variables you can use macros in expressions but you cannot assign a value to them.

The pre-defined macros are generally used to provide easy access to system information like the location of the Windows directory, or the name of the logged on user.