Sonar is one of those areas that is pretty poorly represented in most naval rules. Its generally seen as a device purely for detecting submarines but thats not the case; in many action reports from WW2, especially those fought at night or in reduced visibility the initial contact with, or at least alerting to the presence of, an enemy surface force was achieved by sonar.
Most DDs and many smaler ships had sonar (so did some cruisers and the odd aircraft carrier); the trait I believe represents those vessels with advanced sets.