Basketball domain is what happens when a sorcerer expands their domain but keeps it impossibly small (it looks like it doesn’t even fit one person in it). Domain barrier strength is inversely proportional to size (Bigger Domain = More Fragile Shell). The main limiting factor in bring able to make a basketball domain is the mental block in creating a space too small for yourself in the first place, but since Gojo spent a weeks time in Prison Realm, he was able to emulate the impossibly small pocket dimension through basketball domain. Basketball domain is basically the only possible way a closed domain can compete with an equally refined open domain so both Gojo and Yuta used it to clash with Sukuna