What does 'W/DIO' mean when used for MOSFET descriptions?
That appears to be a description that Farnell generated. The description for that part at Digikey, for example, is just "MOSFET N-CH 500V 18A TO220", and at Mouser it's "MOSFET 560V 18A 223W 270mohm @ 10V".

I can only speculate about what Farnell intended, but maybe they meant "with diode". This is kind of a silly thing to add to the description, because this part doesn't have any integrated diode except for the body diode that all MOSFETs have between drain and source. 

The part's datasheet does contain some characteristic specs for the properties of the body diode, which might make it useful in some applications where you're actually going to use the body diode to carry current.

It might make sense to describe a MOSFET as "with diode" if it had, for example, an integrated ESD protection diode between gate and source.