ATX Riser cards are used with ATX form factor mother boards. The ATX Riser card allows compact PC chassis designs with out expansion slots to provide additional expansion slots. Additional slots are added in the vertical direction by using the Riser card which contains the new expansion slot connectors. A compact PC chassis may be used with an ATX Mother Board, or a larger chassis can be designed using the same Mother Board and a Riser card, providing expansion slots. This standard differs from the others in that it provides expansion slots and not just one slot used for an expansion card, such as a modem card.

The ATX Riser standard requires a single PCI slot connector on the Mother Board, with the addition of a 2 x 11 pin connector in front of the PCI connector. The 2 x 11 connector is also a PCI style connector. There is an 8.35 mm gap between the two connectors. The ATX Riser board will provide either 2 or 3 expansion slots. The basic size of a 2 slot card is 120.85mm x 50.8mm. The basic size of a 3 slot card is 120.87mm x 71.12mm.
Other types of Riser Cards.

In general the riser card has more to do with a compact size chassis, and less about what expansion slot is used.
That is, riser cards are required to add expansion slots regardless of the actual expansion interface.