A113 is a recurring media easter egg created by alumni of the California Institute of the Arts. The number refers to the classroom used by the graphic design and character animation students.
When Nemo and Marlin (his dad) are talking to the sea lions about Dory being sent to Quarantine, the sea lions tell them theres nothing to worry about and show the tags that are located on their flippers.
If you look closely you’ll notice the numbers on their tags combine to say A113.
When Dory finds herself stuck on the truck and asks Hank (the octopus) to help her get back to the ocean, during the following scenes we see the front and rear license plate numbers of the truck are “CALA113”
The “CAL” portion of the license plate number refers to the California Institute of the Arts while “A113” refers to the room number at the college most graphic and animation students now working with the Pixar team used.