According to the IO docs, iodata is binaries or lists of bytes and binaries. Easy to understand when you try:
iex(1)> IO.iodata_to_binary([65,66])
"AB"
iex(2)> IO.iodata_to_binary("AB")
"AB"
iex(3)> IO.iodata_to_binary(<<65,66>>)
"AB"
iex(4)> IO.iodata_to_binary([65,66,"CD"])
"ABCD"
iex(5)> IO.iodata_to_binary([65,66,"CD", <<69, 70, 71>>])
"ABCDEFG"
