Adobe Flex Actionscript Question:

Macromedia Flex 1.5 you can build runtime shared libraries (RSLs) that can be individually loaded, cached, and used by multiple applications.
Use Flex 3 runtime-shared-libraries (RSLs) to reduce the size of your applications and thereby reduce the time required to download the application. RSLs are just SWF files whose code is used as a shared library between different application SWF files. There are two kinds of RSLs, signed and unsigned. Signed RSLs are libraries that are signed by Adobe and may be stored in the Flash Player Cache, which can be accessed by applications from any domain. This means if your application is using a signed RSL, the RSL may not even need to be downloaded if the RSL is already in the Flash Player Cache. The signed RSL may have been put into the Flash Player Cache by visiting another web site that was using the same signed RSL. Signed RSLs have a "swz" extension.
Unsigned RSLs are normal SWF files and are not loaded into the Flash Player Cache. Instead, these RSLs rely on the browser's cache to keep them from being downloaded.