When doing translation, I always look up many documents to decide the translation.
In Voukoder, the alpha channel size has 3 options: 1bit, 8bits, 16 bits.
But in FFMpeg's documentation, it says
Specify number of bits for alpha component. Possible values are 0, 8 and 16. Use 0 to disable alpha plane coding.
I'm not familiar with ProRes at all, I don't know whether it's a bug or it really can be 1bit. I just found this when I confused how to translate it.