Hi, sorry for the late,
leJOS RT implements some of the RTSJ (Real-Time Specification for Java).
So, the advantage of leJOS RT is to be able to develop an application of lejos using RTSJ.
Go there and read the overview, It will be easier to understand what RTSJ is.
For the moment, I know that there is not a lot of documentation on the leJOS RT project. But I hope it wiil be fixed soon.