The .z64 extension signifies a ROM format, which is the native byte order of the original Nintendo 64 hardware.
While both .u.z64 and .z64 contain identical game data, the .z64 extension is often the preferred choice for several reasons: Why .z64 is Preferred mario kart 64 u z64 better
This means the ROM is a 1:1 bit-perfect copy of the original retail cartridge, free from the glitches, corruptions, or bad data found in "overdumped" or "bad" ROMs. The .z64 Extension (Byte Order): N64 ROMs come in different formats based on byte ordering: (Big Endian), (Byte Swapped), and (Little Endian). The free from the glitches