Hi, a dash/number variation indicates some type of engineering change on a model by the factory. These can be as small as changing the direction of threads on a part or moving minor parts from one assembly to another, all the way to major changes in materials or construction, such as frames, hammers and triggers.
To use your example, a 686-6 will have the frame lock, MIM hammer/trigger/thumbpiece, a frame-mounted firing pin, an integral cylinder stop stud, a round butt frame, etc. None of these are on the 686 no dash, but not all these changes started with the -6. Hope this is helpful.