What is a functional writing?

In terms of literacy, a functional ability to read means not just being able to sound out and identify the written Words, but being able to parse the meaning of the entire written work. For example, when you see statistics of places having a high level 'functional illiteracy,' they don't necessarily mean that those people cannot identify letters/sound them out/read the Words. (Although it can certainly include that! ) It means that their reading comprehension is so limited as to be practically useless.

Another example is that if you are learning a foreign language, you may be able to spell and pronounce a wide variety of Words, but if you are given a book and don't understand the gist of it, or only understand bits, you do not have a functional reading ability.

So in this sense, having a functional reading ability is more about one's ability to understand what they are reading.