After the YP series (Containing only the YP rocket) was finished due to the repeating success of the YP rocket, it was time to do more research into single stage rockets, two stage rockets, and other things such as cameras and payloads. This is where the Fortuna series comes into play. The Fortuna series is divided into three sections. The first section is for further testing of single stage rockets. The second section is for testing of two stage rockets. And finally, the third stage is for testing of rockets with payloads, cameras, and other related things.
EDIT: As of 2016, the third stage of the Fortuna series has been terminated due to recent advancements and new plans.