XSharp.Core contains functions that are strong typed and don't have any reference to xBase datatypes, like date and array, and should be compiled in the Core dialect.
XSharp.RT contains the xBase datatypes and all the functions that work on these datatypes and permit parameter skipping and all the other xBase specific functions.
In both of these libraries there should be all these functions and classes that are common to all xBase dialects like VO, VFP, Xbase++ and so forth.
And then there are the dialect specific runtimes: XSharp.VFP and XSharp.VO, containing the dialect specific implementations.
The separation may create confusion, but has much sense because you can use the Core library even from C# or X# Core dialect, and the RT is the base of all the common xBase functionality. So there should be no duplicate code that is hard to maintain.