The Zone system is mainly a method of getting proper exposures. It involves measuring the brightness of the brightest and darkest important parts of the scene to determine the scene's contrast (brightness range) and then determining the proper exposure for the most important item in the scene by determining the value (Zone) you want it to have in the final print, measuring its brightness, and assigning the appropriate exposure. The development time of the (sheet film) negative is adjusted depending on the scene's brightness range so that the negative will have a normal contrast range. Today's roll films have sufficient latitude that average development will usually produce quite printable negatives for most of the scenes you encounter, although you could make an adjustment for the average scene on the roll. Knowing the brightness range of the scene can still be informative.