A "direct reference" means mentioning or pointing to something or someone specifically without any ambiguity or confusion. It is a clear and straightforward way of referring to something without any need for explanation or guesswork. Full definition
Some of the artists have worked in close collaboration with one another; others make direct reference to the work of earlier generations. (greyartgallery.nyu.edu)
We have many returning characters as well as new ones with direct references to what happened in the first movie. (paulstriptothemovies.com)
Overall, there was a sensation of hugely diverse approaches to art making based upon a lack of direct reference to political concerns surrounding Chinese culture in the twentieth century. (galleryintell.com)