Insertion loss is the reduction in transmitted power through the intended path, while return loss describes how much incident power is reflected at a port. They are related through the network but are not interchangeable and must be specified at the same reference planes, impedance and termination state.
A receiver filter with low return loss may create standing waves and gain ripple even if its forward loss appears acceptable. A well-matched filter can still have excessive dissipative loss, reducing delivered transmitter power or adding directly to receiver noise figure when placed ahead of the LNA. Reject averages that hide a passband edge peak.
Provide passband edges, maximum S21 loss and ripple, minimum S11/S22 return loss, connector planes, temperature, power and unused-port terminations. Verify with calibrated S-parameters and preserve both measured and de-embedded data when fixtures are removed.