A113 is a reference to a classroom at the California Institute of the Arts, specifically a classroom used by the graphic design and character animation students. Many alumni of CalArts went on to work at Disney and Pixar, like the now-controversial John Lasseter, Brad Bird, Tim Burton, current Pixar chief Pete Docter, Henry Selick, and more.
As a result, almost every Pixar movie includes "A113" somewhere on screen as a nod to their shared alma mater. Other animated movies and TV shows have also included A113, like "The Brave Little Toaster."
Here are all the A113s we could find.