The Academy doesn't have any rules for how to categorize whether a performance is leading or supporting, and it is up to the discretion of the studios whether a given performance is submitted for either Best Actor/Actress or Best Supporting Actor/Actress.
ðɪ ɐkˈædəmi dˈʌzənt hæv ˌɛni ɹˈuːlz fɔːɹ hˌaʊ tə kˈæɾɪɡɚɹˌaɪz wˈɛðɚɹ ɐ pɚfˈoːɹməns ɪz lˈiːdɪŋ ɔːɹ səpˈoːɹɾɪŋ, ænd ɪɾ ɪz ˌʌp tə ðə dɪskɹˈɛʃən ʌvðə stˈuːdɪˌoʊz wˈɛðɚɹ ɐ ɡˈɪvən pɚfˈoːɹməns ɪz səbmˈɪɾᵻd fɔːɹ ˈiːðɚ bˈɛst ˈæktɚ slˈæʃ ˈæktɹəs ɔːɹ bˈɛst səpˈoːɹɾɪŋ ˈæktɚ slˈæʃ ˈæktɹəs.