It’s the discount that is applied to a normal Oyster card when you have the photocard
The link to the fares mentions the price for a weekly bus/tram ticket at £12.30 but if you use it as PAYG from Monday to Sunday then it doesn’t cap at £12.30 – I myself have spent £12.70 from Monday to today half expecting my last fare today on the 428 to be 45p but instead charged the normal 85p.