Trays are used in Europe due to the construction of their transmitters. They use plug-in modules to enhance the capability of the transmitter. In our transmitters all advanced features are already part of the software. Example, when you buy a European transmitter is is a basic four channel set-up. If you want to expand to six channels than you purchase a module to do that. When transmitters were built in the USA such as Kraft they redesigned the electronics to put everything into a smaller case.