This is the format Nintendo uses for digital eShop content. Official updates, DLCs (like the Booster Course Pass), and digital game purchases are packaged as NSP files.
When dealing with Nintendo Switch backups and updates in the homebrew community, you will frequently encounter two main file extensions: mario kart 8 deluxe switch nsp xci update 30 verified
Added the final 8 tracks and 4 characters (Diddy Kong, Funky Kong, Pauline, and Peachette). This is the format Nintendo uses for digital eShop content