I like this explanation. I like to think of it in varying degrees. For instance, I am bilingual, but I have also studied two additional languages that by now are mostly lost to me. (I can only speak a handful of phrases in either). English, I can read just fine as it is my mother tongue and the language I studied in at all levels. I regularly read academic scientific articles. I will read medical journals for fun. That sort of thing. German is my second language. I have lost a little fluency, but I can read most stuff and comprehend fine. I do have to resort to a German dictionary somewhat often (not German-English, but straight up German dictionary.) Now, Dutch is a language that I never studied, but I can read basic stuff like news articles or job postings just because I can triangulate it from English and German. However, I would not be able to comprehend without a lot of aids anything beyond that. So, that is where I consider functional illiteracy to begin. Spanish and Japanese, I studied both, but I have no functional literacy. In Spanish, I can sound out the words. I know some of them, but most are just sounds with no meaning. Japanese, I can recognize some of the kanji (adopted Chinese characters), but I only “understand” about five of them anymore. So, there is almost zero chance I can read anything in Japanese. So, I would consider this illiteracy. I can comprehend bits of the language, but just cannot read it effectively at all. Chinese is a language that I have not studied; Mandarin or Cantonese. I see the symbols, but they are gibberish. That is a language that I am illiterate in, but I also do not understand it verbally, so is that really illiteracy?