In the foreign exchange relations with a fraction, do we always have the quoted currency (the “one”) in the denominator?

What do you mean? Like: AED 3.675/$

For example, with PPP: S1 / S0 = 1+I DC / 1+I FC The currency (FC) in the denominator is the quoted currency (FC:DC). I am looking for a general rule to define the currency in the denominator and believe it is true that it is always the quoted currency. Can anyone confirm this?

Yes that is correct. For eg: So if you have $3.345/Euro which is equal to 1Euro:3.345 be sure that the will be in the numerator and euro will be in the denominator which will given the equation as: S1/S0 = 1+I $ / 1+I Euro

Because you are pricing the currency in the denominator.