What is a phonic unit? I tried to find out this notion on the Web, but I did not find any explicit or accurate definition but just others definitions that use this notion.
This is not a standard technical term used in linguistics, though it appears to have been used in an English translation of a French textbook by Ruwet. "Phonic" itself is most generally used in spelling pedagogy, and has to do with speech sounds. It could refer to a phoneme, or an allophone of a phoneme, or a group of letters that corresponds to a phoneme (such as "th" = [θ]).