As I write this blog posting it is possible to add a default Product Category code to your AR memo line as you can also do with a default Tax Classification Code. But, whilst the Tax Classification Code does indeed default in, unfortunately the Product Category does not. Oracle have recognised this as a bug and a fix was scheduled for Release 12 but hopefully it will be moved forward. So if you also have a similar issue then raise an SR and you maybe able to get a data fix to allow you too to default a Product Category from your Memo Line to your Transaction Line.