Example1: 
In Main.as an instance of class MyPNG is added(addchild) and MyPNG is not known at compile time.
1)Flex gets a reference to the class with : ApplicationDomain.currentDomain.getDefinition("MyPNG") as Class;
2)The generated (as3) bytecode is automatically extracted from the flex generated swf and injected into the final swf created by hXswfML : <DefineABC file="Main.swf" />
3)hXswfML creates the class for MyPNG : <SymbolClass id="1" class="MyPNG" base="flash.display.Bitmap" />

Example2: 
In Main.as an instance of class MyPNG is added and MyPNG is known at compile time but is an external class (not compiled into Main.swf)
1)Flex gets a reference to it with : new MyPNG()
2)The generated (as3) bytecode is automatically extracted from the flex generated swf and injected into the final swf created by hXswfML : <DefineABC file="Main.swf" />
3)hXswfML creates the class for MyPNG : <SymbolClass id="1" class="MyPNG" base="flash.display.Bitmap" />

Example3:
In Main.as an instance of class MyPNG is added and MyPNG is known at compile time and compiled into Main.swf
1)Flex gets a reference to it with : new MyPNG()
2)The generated (as3) bytecode is automatically extracted from the flex generated swf and injected into the final swf created by hXswfML : <DefineABC file="Main.swf" />
3)hXswfML does NOT create the class for MyPNG but only links the png asset to the class name: <SymbolClass id="1" class="MyPNG" />

Example4:
In Main.as an instance of class MyPNG is added and MyPNG is not known at compile time because it will be in an external library.swf that will be loaded.
1)Flex gets a reference to it with : event.target.applicationDomain.getDefinition("MyPNG") as Class (default)
2)hXswfML creates the library.swf and in it also the class for MyPNG : <SymbolClass id="1" class="MyPNG" base="flash.display.Bitmap"/>

Example5:
In Main.as an instance of class MyPNG is added and MyPNG is not known at compile time because it will be in an external library.swf that will be loaded
1)Flex gets a reference to it with : ApplicationDomain.currentDomain.getDefinition("MyPNG") as Class; (library.swf will be loaded into the same (System) ApplicationDomain)
2)hXswfML creates the library.swf and in it also the class for MyPNG : <SymbolClass id="1" class="MyPNG" base="flash.display.Bitmap"/>

Example6:
In Main.as an instance of class MyPNG is added and MyPNG is not known at compile time because it will be in an external library.swf that will be loaded
1)Flex gets a reference to it with : appDom.getDefinition("MyPNG") as Class; (library.swf will be loaded into a seperate ApplicationDomain)
2)hXswfML creates the library.swf and in it also the class for MyPNG : <SymbolClass id="1" class="MyPNG" base="flash.display.Bitmap"/>

Example7:
In Main.as an instance of class MyPNG is added and MyPNG is known at compile time but it will not be included in the Main.swf.
1)Flex gets a reference to it with : new MyPNG() (library.swf will be loaded in the same (System) ApplicationDomain)
2)hXswfML creates the library.swf and in it also the class for MyPNG : <SymbolClass id="1" class="MyPNG" base="flash.display.Bitmap"/>

Example8:
In Main.as an instance of class MyPNG is added and MyPNG is known at compile time and also compiled by flex into Main.swf.
1)Flex gets a reference to it with : new MyPNG() (but only calls it when it knows the library.swf is loaded and the asset that is linked to that class name is available)
2)hXswfML creates the library.swf and in it the asset with only a linkage to the classname "MyPNG" and does not generate the class itself(flex already did) <SymbolClass id="1" class="MyPNG" />