Technically, ng:autobind is not a directive; it is an Angular bootstrap parameter that can act as a directive. It must exist in the script used to boot Angular and can be used only one time. For details on bootstrapping Angular, see Initializing Angular in the Angular Developer Guide.
ng:autobind
with no parameters tells Angular to compile and manage the whole page.
ng:autobind="[root element ID]"
tells Angular to compile and manage part of the document,
starting at "root element ID".
<script ng:autobind> ... </script>