Passage: Pulp Fiction has been widely regarded as Tarantino's masterpiece, with particular praise for its screenwriting. The self-reflexivity, unconventional structure, and extensive homage and pastiche have led critics to describe it as a touchstone of postmodern film. It is often considered a cultural watershed, influencing movies and other media that adopted elements of its style. In 2008, Entertainment Weekly named it the best film since 1983 and it has appeared on many critics' lists of the greatest films ever made. In 2013, Pulp Fiction was selected for preservation in the United States National Film Registry by the Library of Congress as "culturally, historically, or aesthetically significant".

Question: Critics have described it as what?
a touchstone of postmodern film