Watermarks "designate the makers or manufacturers" of the paper and "help determine the location and approximage dates of works on paper."
Bartsch & Related Series
"During the 18th century, prints for the first time were systematically classified according to schools and individual artists. Much of this organizational work was devised by the Austrian engraver, Adam von Bartsch, who for over 40 years worked with the print collection in Vienna's Imperial Library. The scholarly catalogues compiled by Bartsch set the standard for subsequent print research.... In literature and sales catalogues, Bartsch's print numbers are still used for the identification of specific prints. Other scholars followed Bartsche's lead." (Swan Jones, p. 223-224)