// child SWF uses parent domain definitions
// if defined there, otherwise its own
var childDefinitions:LoaderContext = new LoaderContext();
childDefinitions.applicationDomain = new ApplicationDomain(ApplicationDomain.currentDomain);

// child SWF adds its unique definitions to
// parent SWF; both SWFs share the same domain
// child SWFs definitions do not overwrite parents
var addedDefinitions:LoaderContext = new LoaderContext();
addedDefinitions.applicationDomain = ApplicationDomain.currentDomain;

// child SWF domain is completely separate and
// each SWF uses its own definitions
var separateDefinitions:LoaderContext = new LoaderContext();
separateDefinitions.applicationDomain = new ApplicationDomain();

// set loader context in load()
myLoader.load(request, separateDefinitions);

The three different LoaderContext domain types

ActionScript 3