Calibre the difference between OR and DFM OR command

The DFM OR statement performs an OR operation on the input layers and includes DFM properties from the input layers on the output layer.
While the OR statement only does an OR operation on the input layers without including/considering DFM properties.

A common application of DFM OR is dividing a polygon layer into component layers according to some criteria, deriving properties separately for the component layers, then using DFM OR to combine the component layers. This process is useful when the property derivations are different for the component layers.