RSL stands for Runtime Shared Libraries, and is a means to externally store assets for Adobe Flash Player applications for reference in multiple applications to save space and bandwidth.
From adobe's docs: http://help.adobe.com/en_US/flex/using/WS2db454920e96a9e51e63e3d11c0bf674ba-7fff.html
One way to reduce the size of your applications’ SWF files is by externalizing shared assets into stand-alone files that can be separately downloaded and cached on the client. These shared assets can be loaded and used by any number of applications at run time, but are transferred only once to the client. These shared files are known as Runtime Shared Libraries or RSLs.
When multiple applications share a core set of components or classes, clients can download those assets only once as an RSL rather than once for each application. The RSLs are persisted on the client disk so that they do not need to be transferred across the network a second time. The resulting file size for the applications can be reduced. The benefits increase as the number of applications that use the RSL increases.
