Declare a function which later can be called with the Call Function action. The f_function_length1 defines the number of bytes that the function declaration takes. All the actions included in this block are part of the function. A function should terminate with a Return action. The value used by the return statement should be the only value left on the caller stack.
Do not terminate a function with an End action
Prior version 6, the Macromedia player would keep all the data pushed in a function as is when the function returned whether there was a Return action or not. This means some functions that worked in version 6 and earlier may not work anymore in newer versions.
Since version 7, it is preferable to use the new type of functions: Declare Function (V7).
This action is applied to the current target. This means you are indeed creating a function member that gets attached to the current movie (sprite).