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From my hobbyist point of view one would use a capacitor to allow AC (audio or RF) to pass yet block DC - more or less disconnecting or de-coupling one part of a circuit from another. This may be an oversimplification.
De-coupling capacitors generally serve the purpose of providing low impedance at the frequency of interest. After all, they are capacitors. They can have a variety of applications. For example, in an opamp circuit operating at high frequencies, one might place a de-coupling capacitor on the each of the supply pins so that the fast moving current sources inside the part can provide the currents they need with little voltage drop on the supply pins (PSRR is improved). Another way of looking at it is the de-coupling capacitor holds up the supply voltage during the fast edge of the current inside the part. It is important that the de coupling capacitor be of a high quality. Aluminum electrolytic types will not work well because of their high ESR (causing a voltage drop across all frequencies). Series inductance also degrades performance for the same reason(but it is frequency dependent)
For these reasons, de-coupling capacitors are also sometimes called bypass capacitors because they effectively provide the fast moving supply current at the frequency of interest (thus decoupling the circuit or "bypassing" the normal power supply path).
They are not exclusively used in RF circuits. They are useful anywhere a fast edge of current tries to develop. In otherwords, I could design a 1 Hz oscillator with 100ps rise times on the edges. My circuit only changes state once every 1/2 second - BUT the edges transitioning in 100ps. I would need very good decoupling capacitors in the circuit where 100ps current pulses try to develop.
Determining whether or not you need one and if so where at, is not a trivial task. I have seen so many "lazy-man" designs that place them just because when they are actually not need. Or worse, they do not have them where they are needed.
I suspect the RF circuit you are looking at uses the decoupling capacitor to provide a low impedance at the RF frequency(ies) of interest at that location in the circuit. If it is connected to a supply rail, it is to provide an "AC ground", if it is connecting aplifier stages and used to block DC then it is probably not chosen as a type that is normally used for bypassing.