When you are prompted to choose size for your prints and posters, you'll see two very similar options: A3 WHITE and A3 POSTER. 

Although both A3 WHITE and A3 POSTER correspond to a print on standard A3 paper (11.7 x 16.5 inches, 29.7 x 42 cm), when you choose A3 WHITE your print will print as is. On the other hand, when you select POSTER A3, your image will have an enlarged version of an old book page superimposed on it. 

A3 POSTER and A· WHITE size option comparison