Usually, "nch" is a generic ("baseband") MOSFET, while something with "_rf" suffix is a pcell with a fixed layout, well-characterized (and more accurate) compact model, good isolation (guard rings, etc.), very large area (vs "nch" devices), etc.
Many people prefer to use "nch" instead of "rf" devcies (for the same applications) for the sake of smaller area, and easier customization (even though accuracy for models and simulations is not as good as for "rf" devices).