If the “Convert selection to symbol for tween” dialog box appears, click OK to convert the selection into a movie clip symbol.
If the tweened object was the only item on the layer, Animate converts the layer containing the object to a tween layer. If there are other objects on the layer, Animate inserts layers to preserve the stacking order. Animate places the tweened object on its own layer.
If the original object resided in only the first frame of the Timeline, the length of the tween span is equal to one second in duration. If the original object was present in more than one contiguous frame, the tween span contains the number of frames occupied by the original object.