The Storage Wing is a location on the Severed Floor referred to by Irving when he explains to Ms. Casey that Mark “took [Helly] to look at pen caps” while Ms. Casey was visting Distribution Supply. When asked where precisely, Irving says, “Just at the end of the Storage Wing.” We do not see the location on screen, nor do we know what (beyond pen caps) might be stored there.
Mark and Helly never actually visit the location, as Mark proposed an excursion to see the “extra pen caps” only as a ruse to take Helly on what he later calls a “mental health walk.” It is not clear whether Ms. Casey attempted to or did reach the Storage Wing, as she appears to encounter Mark and Helly by chance while they are walking through a partly lit hallway. The three then appear to head back in the direction of Macrodata Refinement.
Not to be confused with the Storage Closet.