The certification requirements for icing are in CS 25.1419. This will refer you to Appendix C, which gives the engineering standards for icing.
Figures 1 and 4 of Appendix C will detail the envelopes. Note that the x-axis shows mean effective droplet diameter. For the continuous maximum condition in Figure 1, the envelope ends at 40 microns. For the continuous intermittant condition in Figure 4, it ends at 50 microns.
The World Meteorological Organization defines freezing drizzle as droplets of 0.2 to 0.5 millimeters in diameter, and freezing rain as 0.5 millimeters and above. Supercooled Large Droplets (SLD) are defined as droplets larger than 50 microns, or 0.05 millimeters.
So ZL and ZR exceed the envelopes specified in Appendix C. These envelopes are not true certification standards, but rather engineering standards used to define a representative condition. This allows the manufacturers to argue that certification under Appendix C does not prohibit operation in ZL/ZR, and this is how the FAA Flight Standards Service issues Operations Specifications which allow takeoff in light ZR, etc. However, the FAA Aircraft Certification Service has taken the opposite position for several years. They do not believe that aircraft should operate in conditions which so obviously exceed the design parameters generally used.