I - ignore file mode.

i - Ignore.: If used with the 'b' option, the file is accessed as a random file even if created sequential. If used without 'b', the file is accessed "asis"; that is, it is accessed sequentially or randomly depending on the file. The option combination "li" is not valid.
Files in the AFT cannot have their mode changed from random to sequential or vice versa. The "i" option is often used in conjunction with "zi".