The .z64 extension refers to a specific type of Nintendo 64 ROM dump. While the data inside the game remains the same regardless of the extension, the "endianness" or byte-ordering differs:
This is the native format for the Nintendo 64's CPU. It matches how the console naturally reads data, making it the preferred format for modern emulators and preservationists. super mario 64 rom z64 usa
The Ultimate Guide to Super Mario 64: The .z64 (USA) ROM Format super mario 64 rom z64 usa