The identifier is a 32 bit (4 byte) unsigned integer, usually specified as a 8-digit hexadecimal number with a 0x prefix.
In most cases, the first three bytes are the ASCII representation of some letters while the fourth byte is a low number.
For FFS the letters are DOS (D = 44, O = 4f, S = 53) while the last byte represents the flavour (00 = OFS, 01 = FFS, 03 = FFS INTL, 05 = FFS INTL with DirCache).
In your case it is U= 55, N = 4e, I = 49.
In an AmigaDOS shell window you can use ARexx to convert from hex to ASCII and vice versa:
1> rx "say c2x(UNI)"
554e49
1> rx "say x2c(554e49)"
UNI