Objective-C does not support ivars on classes, and does not support adding ivars in categories. You can then imagine my surprise when I found programs that claim that they have class properties, defined in categories, that are backed by ivars.
The Swift compiler appears to co-opt the ivar field to put the name of the variable that holds the value of a static property, even though that variable is not an ivar.
then inspect it with your favorite disassembler. You can see that the NSFormatter (test) category defines an "iso8601" class property with the type string "Tq,N,Viso8601" (data type signed long long, nonatomic, backed by ivar called iso8601).