Entropy coding.
Huffman entropy coding tables can be fixed or custom. Custom typically improves compression by a few percent. Custom tables need to be included in the compressed data so that they can be loaded into the decoder entropy tables.

Can use Huffman or Binary Arithmetric entropy coding.

Adaptive coding uses conditional probabilities and ‘contexts’ to improve entropy coding efficiency. (page 147 [2])

Custom Huffman tables can give an average of 5% better compression over fixed Huffman tables. Arithmetric coding can give an average of 13% better compression over fixed Huffman tables. [2]