Its my understanding that stenographers like court recorders often have shorthands for common phrases, so that they can represent a phrase like "ladies and gentlemen of the court" with a single chord. the representation of the chord as a symbol would normally be done using a combination of letters, though.
signs sometimes convey ideas complex enough that it might require a sentence to convey the idea, although at that point a word could often come to do the same. as an example, for someone unfamiliar with magic terminology
basically means "turn this card sideways", but in addition to creating a symbol to describe that sentence, they also adapted the word tap to mean the same thing, so the idea can now be described with a single word just as easily.words and letters or words and syllables are probably categorically different in the sense that words typically represent entire ideas whereas letters and syllables are typically meaningless on their own and just used to construct words, which then represent ideas. A sentence is then a combination of ideas presented in sequence that conveys a more complex idea, but there's no reason any arbitrary sentence couldn't be represented with a single word (or a single symbol). Realistically people can probably only remember or use a limited quantity of symbols though, and so in a system where symbols represent ideas only the most commonly expressed ideas will become symbols, with the less commonly expressed ideas probably being expressed via a combination of ideas represented by symbols.
what arggh said reminded me about the galifreyan language from doctor who, which basically just ciphers roman characters into a magic circle type shape