It just means that you have the ability to select an output power range in gross amounts, that then allows you to adjust more finitely with improved resolution. So your adjustable output can range from 0-150V, or select your adjustment range to be 0-300V, or you can select it to auto-range. i.e. you set an output voltage and it automatically changes to the appropriate range. The reason you would want to change the range is because your accuracy is +-0.2% of Full Scale. So if your scale is 0-300V all the time, then it can be off by 0.2% of 300 or .6V and if your output is 24V, that's a lot (2.5%). But if you move your scale to 0-150%, the error can only be 0.2% of 150, or .3V so you get better (+-1.25%) accuracy on that 24V output.
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