This has to do with combining several double and single sided fpc flexible printed circuit. These multiple layers are dependent on shielding and/or surface mounted technologies and complex interconnections in order to keep them together.
This type of flexible PCB offers the most effective and efficient solution regarding design challenges. As flexible PCB manufacturer knows that, you will therefore need them to:
Shield applications
Power ground and plane applications
EMI/RFI shielding Control impedance
Route the density and layer of a circuit that cannot be done on just one layer
Increase a circuit’s density