The following image shows a simple example that has several extra conductors on the wave port plane.
This port plane has four metal traces and a ground plane, and only one trace has been assigned a port. In this system, we can reasonably assume the far left conductor is not important to the sourced mode. We will define custom extents for the port to exclude this extra conductor. The first step is to draw a rectangle on the port plane, where the boundaries of the rectangle surround the port and all relevant conductors. Then apply the port attribute to the rectangle (note that the port is also applied to the end of trace as shown in the earlier image). The resulting geometry looks like this.
When the same wave port attribute is applied to both a conductor and a dielectric in a geometry, RF3p defines custom extents for a wave port that's assigned to a trace. When custom extents are defined in this manner, the wave port plane extents as far as the boundaries of the rectangle, instead of including the entire port plane. The next image shows a port mesh annotation on the same geometry, to demonstrate the extent of the wave port plane in this case.