A lumped port induces a voltage difference between two conductors. This voltage is applied during the full simulation, eliminating the need for the preliminary Ports Only solve. This can significantly simplify and speed up your simulation. The cost is in accuracy and parasitics: the wave port is intrinsically free of parasitics, and the lumped port is not. For many applications a simple voltage differential is sufficient, and in those cases you should use a lumped port.
Every lumped port definition needs a voltage reference point, so the port must be defined with either a corresponding negative terminal or an explicit ground reference (patch). In a differential port, the positive and negative terminals are the points that are chosen for the voltage difference. In all other lumped ports, one terminal is on the port, and the other is on the grounding material, as determined by the explicit ground reference. Any symmetry planes in your geometry must bisect the lumped port and its patch. In the case of differential ports, the symmetry planes must bisect all terminals of the port.
RF3p defines each lumped port with a series of integration paths from the port terminal to the ground reference (or the negative terminal, in the case of a differential port). Each integration path defines a circuit element with resistance given by n⋅50 Ω, where n is the number of integration paths. The port's voltage differential is induced by applying source currents along these paths.