You are almost correct, they will in all likelihood be instances of ```DimCoord```, but it's possible to provide an AuxCoord with a matching definition i.e. name etc. and the code will and should still work. If @shoyer makes the change I propose below, you could even use an ```iris.coords.CoordDefn```. I'd be tempted to leave the current wording for the argument type and add to the description to make it clear that the key criteria is that they correspond to dimension coordinates of the cube.