Quotes from Challis Hodge’s article Semiotics: A Primer for Designers (thanks, Erik!):

Semiology … aims to take in any system of signs, whatever their substance and limits; images, gestures, musical sounds, objects, and the complex associations of all these, which form the content of ritual, convention or public entertainment: these constitute, if not languages, at least systems of signification [emphasis mine].

Structuralism is an analytical method used by many semioticians. Structuralists seek to describe the overall organization of sign systems as languages. They search for the deep and complex structures underlying the surface features of phenomena.

… Semiotics and the branch of linguistics known as Semantics have a common concern with the meaning of signs. Semantics focuses on what words mean while semiotics is concerned with how [how?] signs mean. [Emphasis mine.] Semiotics embraces semantics, along with the other traditional branches of linguistics [such as syntactics/syntax (the formal or structural relations between signs) and pragmatics (the relation of signs to interpreters)].

A Text is an assemblage of signs (such as words, images, sounds and/or gestures) constructed (and interpreted) with reference to the conventions associated with a genre and in a particular medium of communication [emphasis mine]. …

[The founder of semiotics, Ferdinand de] Sassure made what is now a famous distinction between language and speech. Language refers to the system of rules and conventions which is independent of, and pre-exists, individual users; Speech referes to its use in particular instances. … According to the Saussurean distinction, in a semiotic system such as cinema, any specific film is the speech of that underlying system of cinema language. [My emphasis.]

Semiotics indeed seems relevant to what I have thought of as constrained universes of expression. Perhaps an expressive universe can be seen as a semiotic system, but I suspect it goes beyond that.