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It is perfectly clear to me now.
The 'Text File Settings' > 'Detect Key By' does not affect the transpose popup on my devices at all, it always comes up as 'C' when it first pops up. When I click(ed) on the 'original key'/'start key' button a new popup appears which allowed/allows me to select the initial key of the piece to be transposed. It had/has no association with what the key is/was on the working document. If the 'Detect Key By' did work in this instance, then the 'original key' concept would work as a method to return the document to an unaltered state. As it was/is, it did nothing more than initiate another popup that allowed setting the start/reference point for the transpose function. The current the setup, to me, very clearly says, "Set the key your document shows using the 'start key', which brings up the list of keys from which you select the desired key. It then returns you to the initial popup so you can do your transposition. The only way it could be made clearer currently, imho, would be to have a '?' instead of the 'C' when the popup first appeared. If/when the 'Detect Key By' works then other things can be done.
The idea of a button to return to the initial setting would be a time saver over just clicking through all of the transpositions to the starting point or just redoing the transposition. It might be useful to save both the original document and the transpositions I would think. [I'm one of those who rarely discards anything. ]