Glenn Morita writes:
The difference between insignificant noise and significant noise is the degree to which the noise affects the operation of the circuit in question.
For example, a switching power supply has a significant amount of output voltage ripple at 3 MHz. If the circuit it is powering has a bandwidth of only a few hertz, such as a temperature sensor, this ripple may be of no consequence. On the other hand, if the same switching power supply powers an RF phase-locked loop (PLL), the result could be quite different.
Understanding the sources of noise, their spectral characteristics, noise reduction strategies, and the sensitivity of the circuits in question to this noise is crucial to successfully designing a robust system.
This application note also attempts to clarify the difference between power supply rejection ratio (PSRR) and internally generated noise, and describes how to apply the data sheet specifications for each parameter.
Noise Sources in Low Dropout (LDO) Regulators - [Link]