In addition there are specialist SpotExchanges that offer a wide range of within day and day ahead products, traded in two main ways:
Continuous spot trading much like any other OTC or Exchange trading
Day Ahead Auctions - with a fixed close
At the close of Day Ahead trading many SpotExchanges publish Day Ahead Settlement prices based on the auctions and/or a particular trading period in the current day, or provide these prices to a third party who publish a Day Ahead settlement price