To use a downloaded save file on a PS4 or PS5, you cannot simply drag and drop the ZIP onto a USB. PlayStation saves are encrypted to a specific PSN Account ID. A USB Flash Drive (formatted to FAT32 or exFAT).
The term "CUSA18581" appears to be a game identifier used by Sony for PlayStation games, suggesting that this save game is intended for a specific version of Elden Ring on the PlayStation platform. The exclusivity of such a save game, distributed in a zip file, could be related to platform-specific content, beta saves, or special edition bonuses. elden ring save game cusa18581 zip exclusive