A frequency-locked loop is an example of a control system using negative feedback.
Frequency-lock loops are used in radio, telecommunications, computers and other electronic applications to generate stable frequencies, or to recover a signal from a noisy communication channel.
Because it tries to do less, an FLL can acquire lock faster and over a wider range than a PLL.
Sometimes the two are used in combination, with a frequency-locked loop used initially until the oscillator frequency is close enough to the reference that a PLL can take over.
Advanced applications can use both simultaneously, creating what is called an "FLL-assisted PLL" (FPLL).