Are you maybe thinking of the options in After Effects? At this point, I wouldn't use AVI for anything anymore, ProRes 4444 with Alpha is going to be a much better, more modern, and cross-platform option. As you mentioned, it sounds like you found the option to choose ProRes 4444 with Alpha, which gives you the depth options of "8/16-bpc + alpha," but the "RGB + Alpha terminology is used in AE (see attached screenshot).
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