You can use software timing or hardware timing to control when a signal is generated. With hardware timing, a digital signal, such as a clock on your device, controls the rate of generation. With software timing, the rate at which the samples are generated is determined by the software and operating system instead of by the measurement device. A hardware clock can run much faster than a software loop. A hardware clock is also more accurate than a software loop.
In NI-DAQmx, select hardware timing with the Sample Clock Timing function/VI or by setting the Sample Timing Type attribute/property to Sample Clock. If you do neither of these things, or you set the Sample Timing Type attribute/property to On Demand, you are selecting software timing.