Previously, the ALD or Advanced Leak Detection module was used to connect the water sensors to your APEX system.
The new LDK or Leak Detection Kit utilizes the new FMM or Fluid Monitoring Module to connect the water sensors. The FMM also accepts the flow meters from Neptune Systems making it a more universal module.
Functionality wise, it is pretty much the same. The only change is that you can now monitor flow rates and use the water sensors via a single module, the FMM.