The first requirement assures you contaminant reduction claims are true. The second assures you the system does not add anything harmful to the water. The third assures you the system is structurally sound. The fourth assures you advertising, literature and labeling are not misleading. And finally, the fifth assures you the materials and manufacturing process used don't change, meaning consistent product quality over time.
All distillers go through the process of distillation.
One brand distiller will not significantly make purer water than another brand. When water boils, it is first converted to wet steam and then later converted to dry steam. Contaminates won't rise with dry steam but will rise with wet steam. A longer distance between the boiling chamber and the condensing coil produces more dry steam, resulting in purer water. Pyrex Glass is the cleanest material to heat water without adding contaminates back into the water.