<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
			   xmlns:s="library://ns.adobe.com/flex/spark" 
			   xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="300" minHeight="300" 
			   xmlns:assets="assets.*">
	
	<s:VGroup horizontalCenter="0">
		<assets:bubble id="fxgBubble" width="100" height="100"/>
		
		<s:Group >
			<s:Group >
				<s:Rect x="136.145" y="229.522" width="173.913" height="156.521">
					<s:fill>
						<s:SolidColor color="#E30613"/>
					</s:fill>
					<s:stroke>
						<s:SolidColorStroke caps="none" joints="miter" miterLimit="10"/>
					</s:stroke>
				</s:Rect>
			</s:Group>
		</s:Group>
	</s:VGroup>
</s:Application>

http://www.digitalwks.com/blogs/fabio-belga/2011/06/28/understanding-fxg-files/

Understanding FXG files

files, Flex

MXML