The multi layer flex circuit refer to a flex circuit with more than 2 layer circuit layers, 4L -8L are normal stack up. Multi layer flexible circuit made of polyimide, is about
How to Avoid Fracturing Traces in a Flexible PCB Design
4 Layer Rigid-Flex PCB with 2 Layer Flex PCB by Rigiflex Technology
Production of a flex-rigid pcb | Würth Elektronik: Printed Circuit Boards > Products > Flex-Rigid > Production Technique