A single pole always produces 20dB/dec. e.g. one resistor and one capacitor. It means that for a 10 times increase in frequency you get a 10 times reduction in the signal. As the reactance of a capacitor is inversely proportional frequency, that makes sense.
This is true (far above the 3-dB corner frequency) only for a first order system - that means, if the slope is caused by one capacitor only. If the frequency is high enough and more capacitive effects come into the game, you will observe always a slope of -40 dB/dec and higher.