Taxation classes (tax groups, Lohnsteuerklasse aka Steuerklassen)

class I = single
class II = single parent (living alone with the child/children)
class III = married and spouse has no income or lower income[citation needed]
class IV = married and similar income to spouse
class V = opposite of class III, ie this is the class, your spouse has, if you have III
class VI= for a second job or for deduction without proper employee information