Kostas, if you read the tech notes, it is explained:
***For this unit to operate properly with ONLY Futaba PCM***
***and ONLY Hitec RCD receivers, user must get an amplifier from Ram.***
This is a FREE plug and play module with connectors to simplify installation.
***Send the header card from this package and $4.00 to cover postage to Ram.***
***This module is not necessary for any other receivers.***