Normally ribs can be smoked based on time alone. The standards for baby backs are 2-2-1 and for St Louis style 3-2-1. What it boils down to is that the first number (3 or 2) is the amount of time the ribs are in the smoker over smoke (usually at 225° to 250°) and most of us will spray/spritz them with something to help keep them moist after the first hour, I prefer raspberry/cranberry juice.
After those three hours you will take them out and wrap them in foil with whatever liquid you sprayed them with or perhaps with some butter and honey. They will be back in the smoker at the same temperature for two hours (the second number, 2).
Finally is the last number, 1. This represents the amount of time the ribs are removed from the foil and set back in the smoker (still at 225° to 250°) for one final hour just to firm them up.
If you are the type that likes to add a sauce or glaze to your ribs, you can now take them to the grill and allow the sauce/glaze to caramelize adding a whole new dimension to your ribs.
I hope that helps answer your question.