  Reconfigurable intelligent surface (RIS) has shown promise in providing
apparent benefits in wireless communication and positioning. Most of the
existing research focuses on the ability of RIS to adjust the direction of
propagation. In this paper, we present another application based on RIS, named
multipath labeling, which intends to inject a label on propagation paths
through the RISs. Each labeled path contains spatial knowledge between the RIS
and the receiver, thus opening the door for sensing the surrounding world by
RISs. The critical challenge is how the labeled paths can be extracted and
distinguish from other paths, especially with multipath effects. To address
this challenge, we present a complete labeling procedure consisting of channel
flipping, parameter extraction, and RIS association. Simulations under a
practice ray-tracing model reveal the feasibility of the proposed labeling
technique even under the sub 6GHz channels with severe multipath effect. We
also apply the multipath labeling technique to localization, which assists user
equipment in obtaining its precise location through only a single base station.
