Expressio unius est exclusio alterius
to express one thing is to exclude the other
Where the phrase comes from
A canon of interpretation used on statutes and contracts. A list that names A and B can imply that C was left out on purpose.
What lawyers are pointing at
The canon is a guide. It yields when the text shows the list was a set of examples ('including'), when a later clause covers the leftover case, or when the exclusion would defeat the purpose the instrument states. Lawyers who treat the canon as automatic over-read lists. The honest use is to ask whether the drafter was closing the set or illustrating it.
The same shape in code
An allow-list of HTTP methods is a closed set. GET and POST on the list do not smuggle in PATCH. If you meant examples, write 'including' and keep a default. A middleware that invents the missing method because the canon feels harsh has stopped reading the list.
A concrete picture
A lease says the tenant may keep 'one dog or one cat.' A pony is not a close case of dog or cat. The list did the excluding.