In the Typeface Information of the NSFontDescriptor Class Reference, it says, “Expanded and condensed traits are mutually exclusive”. But it does not say that vertical and italic traits are mutually exclusive.
Does it mean that there may be fonts which have both vertical and italic traits, perhaps like upright italics? Also, are they jointly exhaustive traits unlike expanded and condensed traits, i.e., are all fonts vertical if not italic?
I couldn’t find a single font having a vertical trait to test and see what exactly it means.
I’m guessing it means fonts which are designed to be used for text flowing from top to bottom instead of left to right as against upright fonts which I wrongly thought it to mean as @djangodude rightly pointed out.
Hence, vertical and italic traits must be totally independent of each other.