The specs are:
1) The intro should be built with a single movie clip, that can contain other movies inside. So the essence is that the main timeline can have only 1 clip which will host all the rest of the Flash movie.
2) The first frame of the main movie clip should define the following constant:
intro = 1;
3) The last (the one that should finish the intro) frame has to host the following directive:
intro_begin = intro;
4) The intro should have 18 FPS framerate.
5) Show.kit Intros do not support buttons. If you need to add something like "Skip Intro" you need to use movie clips for that.
6) Only Event sounds are supported (No stream sounds).
7) Shapes’ Fill style property cannot be an image.
8) All objects should be present on timeline: Show.kit Custom SWF’s do not have support for Macromedia Flash Library.
9) Main timeline should not use any "stop();" directives.