As far as I know and remember, if using an SL access card on the Stockholm tunnelbanan, a single fare can be bought by just using the card at the gate to enter. But how does that work if one is entitled to a reduced price because of young or old age? Does one have to use the vending machines to get the discount?
When you buy the card you can decide if it is gonna be for full or reduced price. It is also possible to change it in a vending machine or by talking to an agent in the subway or on the bus. It is not possible to do over the web.