Yes, both. When you ask for 1 sample from the LabJack you get the voltage at that moment. If the signal is DC, you expect multiple samples over time to return the same value. If the signal is AC, the samples are taken at different points of the waveform and thus will return different values. If you want to acquire the waveform for an AC signal, you simply acquire multiple samples over time, which is often done using stream mode. Then if wanted you can do math on the waveform to determine parameters such as max/min, RMS, or frequency. On T-series devices you can use the AIN-EF system to acquire multiple samples over time, do some math, and return various values that describe the multiple samples (RMS, Peak-to-Peak, Period, Average, Max/Min).