Right now I am doing a clean install of Catalina. I have erased the Macintosh HD and the Macintosh HD -Data. When I tried to install the OS it gave me these two choices again. I couldn’t find anything on apple’s website on this choice of drives. I chose now the Macintosh HD because it is defined as the system drive but I am not sure. I would be happy if you provide some information or a reference about this.