From the Wikipedia page  which includes examples.
In telecommunication, a commercial code is a code once used to save on cablegram costs. Telegraph (and telex) charged per word sent, so companies which sent large volumes of telegrams developed codes to save money on tolls. Elaborate commercial codes which encoded complete phrases into single words were developed and published as codebooks of thousands of phrases and sentences with corresponding codewords. Commercial codes were not generally intended to keep telegrams private, as codes were widely published; they were usually cost-saving measures only.
When telegraph messages were the state of the art in rapid long distance communication, elaborate commercial codes which encoded complete phrases into single words (commonly five-letter groups) were developed. A "dictionary" of such "words" is a codebook. For telegraph offices that would not accept messages with non-words such as AYYLU, codebooks were compiled with only real words as codewords, sometimes using words from multiple accepted languages to expand the supply even though all the plaintext phrases were in one language.
Many general-purpose codes, such as the Acme Code and the ABC Code, were published and widely used between the 1870s and the 1950s, before the arrival of transatlantic telephone calls and next-day airmail rendered them obsolete. Numerous special-purpose codes were also developed and sold for fields as varied as aviation, car dealerships, insurance, and cinema, containing words and phrases commonly used in those professions.
Such commercial codes became obsolete in the late 20th century. They were replaced by much simpler (although admittedly more long-winded) codes such as Morse Code Abbreviations and Ten-code and Q code, and also by more compact automatic data compression algorithms.
In this context, "commercial code" (used purely to save cable costs, where the people communicating didn't care if anyone else could read their messages) is distinguished from similar "secret codes" used in cryptography.