The main difference between the cuts of fish or seafood used in nigiri and sashimi is the thickness of the slices.
In nigiri, the slice of fish or seafood is typically placed on top of a small ball of vinegared rice, so the slice is usually thicker than the slices used in sashimi.
Sashimi, on the other hand, consists of thinly sliced raw fish or seafood that is typically served with soy sauce and wasabi for dipping. In addition to the difference in thickness, the cuts of fish or seafood used in nigiri and sashimi may also vary in terms of their shape and size.
Nigiri slices are usually rectangular or oval in shape and are typically smaller than sashimi slices. Sashimi slices, on the other hand, can vary in shape and size depending on the type of fish or seafood being used and the preference of the chef.