RAction classes handle user interaction with a document. An action reacts to mouse clicks, coordinates that are being entered, etc.
Example for RAction implementations: File - Open, Zoom in, Draw line from two points and any other QCAD tool.
Actions are the 'glue' between the user inputs and the QCAD application.
ROperation classes encapsulate a modification that can be applied to a document.
Such an operation might add objects (entities, layers, blocks, ...), delete objects or modify objects.
Operations are a low level concept and would also exist in an application without user interface.