The ExternalInterface.available property indicates whether the current Flash Player is in a container that offers an external interface. If the external interface is available, this property is true; otherwise, it is false. Before using any of the other functionality in the ExternalInterface class, you should always check to make sure that the current container supports external interface communication, as follows:

if (ExternalInterface.available)
{
    // Perform ExternalInterface method calls here.
}

The ExternalInterface.objectID property allows you to determine the unique identifier of the Flash Player instance (specifically, the id attribute of the object tag in Internet Explorer or the name attribute of the embed tag in browsers using the NPRuntime interface). This unique ID represents the current SWF document in the browser, and can be used to make reference to the SWF document--for example, when calling a JavaScript function in a container HTML page. When the Flash Player container is not a web browser, this property is null.