The Nikon Z 9 features two memory card slots. Each slot supports the extremely fast CFexpress Type B cards or the slower XQD cards.
The CFexpress 2.0 standard is supported but not the latest 4.0 standard. CFexpress 4.0 cards can still be used but you won't see any speed benefits in-camera.
The card in Slot 2 can be used for overflow or backup storage, for separate storage of NEF (RAW) and JPEG pictures, or for storage of duplicate JPEG pictures at different sizes and image qualities; pictures can be copied between cards.