Generally, a symbol of probability p contains lg(1/p) bits of information: 1 bit if p=1/2, 2 bits if p=1/4 etc. In Huffman you directly assign a concrete bit se

Generally, a symbol of probability p contains lg(1/p) bits of information: 1 bit if p=1/2, 2 bits if p=1/4 etc. In Huffman you directly assign a concrete bit sequence to every symbol - it is perfect if their probabilities are powers of 1/2, but generally is not true: requires approximations, leading to a suboptimal compression ratio. Accurate entropy coders like arithmetic/range coding or ANS family can directly work on symbols of general probabilities: containing a non-integer number of bits. It has to finally produce complete bits - their fractional number is handled by the state of the coder - kind of a buffer containing a fractional number of bits. Complete bits are produced as soon as they accumulate.