My question is motivated by the media frenzy around the publication of an autobiographical memoir by Prince Harry, the second son of our current king, Charles III. The premature release of the Spanish translation of the work reveals that the title has been rendered as En la sombra while the English title is to be Spare. I thought I knew a word for spare and the DLE confirms that recambio does indeed, amongst other things, mean
Pieza destinada a sustituir en caso necesario a otra igual de una máquina, aparato o instrumento
I note though that this seems to refer to spare parts for machines and similar objects. The sense in which the title Spare was chosen is believed to relate to a comment made by his father after Harry's birth that now he had "A heir and a spare". As the second son Harry would only accede to the throne in a fairly unlikely set of circumstances.
So my question is whether there are other words for spare that could have been used here. I am not complaining about the work of the translators, they have read the book and must feel that En la Sombra conveys what they read in the book and they know how Spanish readers would understand their choice better than a more direct translation.