The sample rate, specified in samples per second (S/s), is the rate at which a signal is sampled and digitized by an analog-to-digital converter (ADC). According to the Nyquist theorem, a sample rate at least twice the highest frequency of the signal produces accurate measurements if the analog bandwidth is wide enough to let the signal pass through without attenuation. A higher sample rate captures more waveform details for the time domain measurements.
The following figure illustrates a 1 MHz sine wave sampled by a 2 MS/s ADC and a 20 MS/s ADC. The faster ADC digitizes 20 points per cycle of the input signal compared with 2 points per cycle with the slower ADC. In this example, the higher sample rate more accurately captures the waveform shape.
The NI ELVIS III oscilloscope's maximum sampling rate is determined by the speed of the onboard sample clock(100 MS/s). However, you can achieve other sampling rates by the decimation of the data. In the decimation method, the ADC samples at the rate of the onboard clock and then sends its digital data to a decimator that essentially discards samples at a specific interval to achieve slower effective sampling rates. The valid sampling rates are always an integer divisor of the onboard clock. For example, if the onboard clock is 100 MHz, but you want to sample at 25 MS/s, you must use decimation. The decimation method discards all data except for every fourth data point to achieve exactly 25% of the maximum sample rate.