Off the top of my head, I'd define a master format as something that can be completely and losslessly converted to other formats as needed.
Well, obviously that never happens. First, there is a loss when the transcriber transcribes from the "display format" of the printed page to the "master format" which doesn't contain formatting information -- but needs to, because at least some of the information in the original book *is* encoded in formatting choices -- and then there is a second loss in converting from the "master format" to the "other format" when there is not a one-to-one match of elements, which means that the conversion has to "fake it." There are also losses when the transcriber does not completely understand, or does not care to completely understand, the "master format." And there are also losses when the writer of the translation tools don't bother to understand all the details of the other format which leads them to "fake it" more than the need to [or less than the need to, depending on the reliability of the actual target] So in practice each and every conversion leads at least potentially to a loss.