Why does the Model 2015 have three methods for computing the frequency of the fundamental?

Why does the Model 2015 have three methods for computing the frequency of the fundamental?

The Model 2015 offers a speed /accuracy tradeoff for the measurement of the signal's fundamental frequency. AUTO operation measures the frequency of the fundamental for every measurement and provides the most accurate distortion computations. ACQUIRE measures the fundamental frequency once and uses the result for future measurements. This speeds up the distortion measurement and is faster than the AUTO mode. The final mode, SET, uses a programmed frequency to make the distortion measurements. This is the fastest measurement mode.