Basically, yes, PPM is the only factor to safe CP water, because PPM stands for 'parts per million' in dissolved solids. That is, if you have 40 PPM water, it has 40 molecules of dissolved solid compounds (salt, minerals, etc.) per every million water molecules. 40 PPM isn't bad.
The only other thing I can think of that would affect water quality is PH, which stands for 'power of hydrogen/hydronium' ions, which is responsible for making the water acidic (PH less than 7) or basic (PH greater than 7). Good, pure water has a neutral PH (7), neither acidic nor basic. I'm not quite sure if the responsible compounds would show up on a PPM test or not, but it's pretty safe to assume that bottled or even low-PPM water has a neutral PH of 7.