Have you checked the ADS help?
qthelp://ads.2015.01/doc/em/Theory_of_Operation_for_Momentum.html
-> Comparing the Microwave and RF Simulation Modes
Momentum RF mode re-uses some of the coupling functions across different frequencies, so that simulation is much faster after solving the first frequency point. RF mode is valid for structures that are small compared to the wavelength ("circuit size smaller than half a wavelength") and do not radiate (or have very little radiation).