Level 31 is the bottom level of Mainframe, it is a feared and dangerous place. Mainframe's less reputable citizens gather there, and some criminal activity goes unnoticed on this level.
Al's Wait & Eat is located on Level 31, it is Mainframe's only source of slow food. Bob had to travel here to buy Dot some slow food, when she was partially erased by a magnet. (The Quick and the Fed)
When a Web Creature arrived in Mainframe, it hid in Level 31. The creature created a small lair on this level, where it hid the Mainframers it abducted; they were kept in large energy draining sacks so the creature could feed off their energy. (Trust No One)