RDML is a high level OO language, used to develop the whole application, both client and server. It also insulates you from the underlying technology which is constantly changing and evolving.
RDML, when compiled, generates code for the target platform, for example C++ for desktop and JavaScript and HTML for web and web mobile.
Why RDML and RDMLX?
When we extended RDML to support components and OO, we called these extensions RDMLX.
We normally refer only to RDML, meaning both RDML and RDMLX.