RCDC is short for Resource Control Display Configuration. Don't ask me who came up with the name or why it got this name, but this is a form. Every form used in FIM to manage any kind of information is defined in what is called an RCDC.
All over the Internet, it is easy to find discussions on how to create and modify the RCDCs. I will give you some examples of typical changes I make to the RCDCs in my customer projects, but for a complete reference on the structure of the RCDCs, refer to http://aka.ms/RCDCRef.
Let's say we want to change how the form we get when looking at a user object looks. You then need to modify the RCDCs as per the following steps: