It is known fact that in AX there are quite a lot of kernel classes, tables, data types, functions, etc. that are not visible in the AOT but can be used from the code. For example, class Args. There is a nice way to get an overview of all this stuff - in the System Documentation node in the AOT. The full list of all kernel objects and functions can be found there. And actually some of them have nice documentation - the one that can be found on MSDN.