There are commands you type in the Rhino command line (like BooleanSplit or Split) but can’t use in RhinoScript and commands you can use in RhinoScript that you can’t type on the command line (like SplitBrep).
Why is this? I find it really frustrating that once you get the sequence of commands right that are needed to accomplish something, you can’t “bake them” into a script.
You can always use commands that have no Rhinoscript equivalent as Rhino.Command. Does that help?
Rhino commands are higher level than Rhinoscript functions - that is to say that in general they do more things and offer more options, as well as user interaction. So generally something like Rhinoscript BrepSplit is covered by Split (which does a lot more than just split breps) but the inverse is not - there is nothing native in Rhinoscript that is the exact equivalent of Split. As Gijs said, if you really need all the functionality of the native Rhino command in a script, you can always program Rhino.Command(“YourCommand”). Otherwise, you can also generally build what functionality you need with native Rhinoscript methods, it just needs more lines of code.