The NGRAVE "Perfect Key" is the 64 character hexadecimal equivalent of a 24 words mnemonic phrase / 256-bit (0s and 1s) master seed.
Hexadecimal means that each character of the key can have one of 16 values (0-9; A-F). The total key consists of 64 characters that together with the 16 possible values makes up a 256-bit key.
NGRAVE came up with the use of this format.