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Driven Modal Solution
Choose the Driven Modal solution type when you want HFSS to calculate the modal-based S-parameters of passive, high-frequency structures such as microstrips, waveguides, and transmission lines. The S-matrix solutions will be expressed in terms of the incident and reflected powers of waveguide modes.
Driven Terminal Solution
Choose the Driven Terminal solution type when you want HFSS to calculate the terminal-based S-parameters of multi-conductor transmission line ports. The S-matrix solutions will be expressed in terms of terminal voltages and currents.
You can use both all the time. Driven Terminal is easier to set up for all conditions, so unless you really want modal s-parameter data, I suggest you use driven terminal. Driven Modal for waveguides may give better results for some cases.