Runtime Shared Library (RSL) is similar to a DLL in Windows. It can be used by multiple applications that uses same code. RSL are of various types like standard, cross-domain, framework RSLs.

Framework RSL can reduce app size and is signed by Adobe. It is secure, can be used with applications made with Flex framework that are deployed as web apps. The framework RSLs are cached by the player. It reduces the app size to around 30% with Flex 4 framework. It reduces the bandwidth, but increases the loading time of the application since the whole library will be loaded.

Generally there is no point in using RSL (framework RSL) with AIR apps. For AIR apps the best way is to choose merged into code option where only the referenced classes are included with the final swf.