The citizens of these cities had a higher position in the estates of the realm than any other subjects.
A similar status was dowry town; these were a source of income for the Queen.
Originally, this meant that each citizen had to work the kings land a certain number of days each year, but this was soon replaced by a cash payment.
If the applicant purchased a home, or lived in the city for a long time, or could prove "good" ancestry and a decent family life and, if he had been a subject, could show a release certificate, then the applicant had a chance to be admitted to this privileged class.
Even among the citizens, there was a hierarchy, which was reflected in income, the order at the Last Supper and the seat in the church, but also at the location of the house.