MovieClip Linkage - Export for Actionscript
In the Library when you right click on a movieclip and select linkage, then you will get a symbol properties window.
In the linkage section you will select an ID name that you can use when you are using attachMovie method.
If you select “Export in first frame” that means the movieclip will be loaded before any preloaders. Thus defeating the purpose of the preloader.
So leave that box unticked if you just want the movieclip loaded when you are using it.
but be aware that if there are any issues say dealing with parent or inheritance and you use this movieclip before a parent clip , then you may run into trouble and it wont work.
If you need to use attachMovie and are not going to drop the movieclip onto the stage, then tick the “Export in First Frame” box or the attachMovie method wont work..
Basically just have a think about what you want and select the appropriate box for your needs.