'With added John Innes'
I've been buying compost today. I understood John Innes to be a formula for compost which makes it suitable for either seed sowing, cuttings, mature plants etc, and it had a corresponding number I.e. John Innes number 3.
When I was shopping for compost, some of the bags said on them 'with added John Innes'. What does this mean? What number John Innes and how can it just be added to other compost?