底定 is literary/formal. Two main flavors: (1) classical sense 'to pacify, to quell' (
天下底定, tiānxià dǐdìng, 'the realm is pacified'), and (2) Taiwanese Mandarin sense 'to be settled / finalized,' very common in Taiwan news for elections, deals, situations. Mainland speakers more often use
尘埃落定 (chén'āi luòdìng) for the same idea.