The following resources document the X10 API:

X10RT is a library that an X10 program uses to communicate between places. There are multiple implementations of X10RT that could be used to execute X10 programs. Details can be found here. There is also an online specification of the X10RT API which may be of interest if you want to port X10 to run on top of another messaging layer.

Here is additional information about environment variables, as well.