[DaveBlake]

IMHO the Kodi wiki doesn't really help much in understanding exactly how the node code works (for example, doesn't explain "filter" vs "folder"). And your example is using "folder". Why is that? could you do the same with "filter" and then create rules?

There is a thread of example nodes here http://forum.kodi.tv/showthread.php?tid=257378, but I agree that the wiki could do with improvement. My understanding is gained initially from trial and error and then digging through the code.

I use folder type cutom nodes because I prefer how they behave compared to filter type or smart playlist rules.
In folder type nodes:

a) there is an optional "* all" entry that navigates to the next level down e.g. all the albums for those artists, or all the songs for those albums.

b) the filtering passes down the levels e.g. showing artists with (songs with) genre=classical, click on artist you see the albums by that artist with (songs with) genre=classical, click on an album and you see those songs with that genre.

With filter type rules they just apply to the top level. So you could select artists with songs you have played once and they get listed, but driling down from such an artist you would se all their albums, and then all their songs not just the ones played once.

The SQL in folder type is also more efficient, but that is just my database brain working overtime, the filter type works well enough.

But yes you can use role in a rule if you want to. In fact if there are other more complecated filtering you want then filter type is the only way.