Rigid flex PCB, as the name suggests, it's combined with rigid PCB and flex PCB, and the rigid circuits are connected with single or multiple flex circuits through plated through holes. Coverlay's are applied to the flexible copper conductor and liquid photo imagable solder resist applied to the rigid PCB areas.
Rigid flex PCB is different with the case of flex PCB with FR4 stiffeners, basically no any conductors or plated through holes(PTH) on FR4 stiffener but only be used with the purpose of increasing strength for specified areas, for rigid part of rigid flex PCB, there are conductive circuit patterns and plated through holes extend through both rigid and flex layers.