DIN wiring duct refers to the specified external dimensions of height and width. These dimensions are based on the DIN 43659 standard, which has since been withdrawn but is still widely used in the industry. DIN sizes include size increments like 20, 25, 75/2, 50, 75, 100, 125 for width and height.
Metric sizes are a different system than DIN. They include more height-width combinations. Both metric and DIN channels use metric – meter-based – units of measurement as opposed to imperial units of measurement.
DIN can also refer to the standardisation of the bottom perforation of the duct. The perforation is used when mounting the duct to the cabinet. The position of the oblong holes used to be specified in the DIN  43659 standard. The standardization is still present e.g., in the current standard DIN EN 50085-2-3 as an example. When looking for a standard perforation according to DIN, DIN EN 50085-2-3 should be used.