Hi Derek
Not all candidate supernovae turn out to be supernovae, for example they may turn out to be novae, Luminous Blue Variable outbursts or other cataclysmic variables in our own galaxy. A follow up spectrum is needed to establish exactly what type of object it is. (For example the redshift measurement can be used to confirm if it is extra-galactic and the width of the lines can be used to measure the velocity of the material flung out in the explosion which is much higher for supernovae.There are also different types of supernovae produced by different mechanisms, for example core collapse of massive stars or the thermo-nuclear explosion of a white dwarf following accretion of material from a companion. These different types of event have characteristic spectral features and are very different from what the spectrum would have been before the explosion.
An introductory paper on the subject is one by Filippenko here
https://ned.ipac.caltech.edu/level5/March03/Filippenko/frames.html
also brief but more up to date this blog entry
https://astrobites.org/2016/12/02/classifying-supernovae/
I have talked more about this and how I am using spectroscopy to confirm and classify supernovae spectroscopically in more detail in a few BAA meetings a couple of which were videoed
https://britastro.org/video/11250/12234
https://britastro.org/video/13862/14769
Also short more up to date piece on the Sky and Telescope website currently
Cheers
Robin