What @GIJW was probably referring to is that Elemental to [other damage type] and [specific damage type] to [other damage type] conversions don’t mix and are multiplicative.
So using an example:
- 50% Fire to Aether
- 50% Elemental to Aether
This would first convert the Elemental (if converting Fire) to Aether leaving 50% Fire Damage remaining to be converted. This in turn will be 50% converted to Aether leaving 25% unconverted Fire Damage. So, in this example, ideally you want to have all Elemental to Aether conversion or all Fire to Aether conversion. Hope that helps!
There is also a predetermined order of conversion from equipment slots as outlined by 
here:
