Hi moonkim,
The integration lines serves in calibrating wave ports. And Wave Ports that are added to a structure must be calibrated to ensure consistent results. This calibration is required in order to determine direction and polarity of fields and to make voltage calculations.
If you are analyzing multiple modes, define a separate Integration Line for each mode since the orientation of the electric field will vary.
You may need to run a ports-only solution first to help determine how the Integration Lines need to be applied to a Wave Port and their direction.