If two SWF files are served from different domains -- for example, http://mysite.com/movieA.swf and http://othersite.com/movieB.swf -- then, by default, Flash Player does not allow movieA.swf to script movieB.swf, nor movieB.swf to script movieA.swf. A SWF file gives SWF files from other domains permission to script it by calling System.security.allowDomain(). This is called cross-domain scripting. By calling System.security.allowDomain("mysite.com"), movieB.swf gives movieA.swf permission to script movieB.swf.