In computer science's object-oriented programming, class variables (static fields) contain information which is associated with all instances of a class. If all tanks have one cannon, then it is not necessary to specify, in an instance variable for each tank object, that each tank has only one cannon; rather, one can simply define a class variable which does this and thus all instances of the class will share the variable.
... out of which 27.3% have children under the age of 18 living with them, 44.2% are married couples living together, 12.2% have a female householder with no husband ...