A low insertion loss system can be achieved by minimizing the number of connectors and through-paths, or by selecting low insertion loss devices for system configuration.
Passive crossovers, like the driver units that they feed, have power handling limits, have insertion losses (10% is often claimed), and change the load seen by the amplifier.
It is most typically applied to the insertion loss of the network, but can in principle be applied to any relevant function of frequency, and any technology, not just electronics.
The next-generation passive wavelength division multiplexing modules that the manufacturers offer low insertion loss, unmatched stability and reliability, high channel isolation and many other features and benefits.