What is the difference between SK protocol and SD protocol on the 5820XL, and can they be mixed?

SK and SD protocol devices cannot be mixed in the same fire alarm system [1].

The primary difference is that SK protocol allows detectors and modules to share the same address (1-99) because the panel distinguishes between device types, while SD protocol requires every device to have a unique address from 1 to 127 (Page 98) [1].