Linguistics & sociolinguistics
Language isn't just what grammar books say it is. It's what people actually do with it — slang, code-switching, tone, and everything a sentence means because of who said it, to who, and where. If a system is going to understand meaning, it has to deal with the social side of language, not just the syntax. For me this is the starting point. Everything else builds on it.
Not communicating is still communicating. The second someone is there — seen, or just able to act — everyone around them responds differently. They respond to that person existing, and to what that person could have done but didn't. Silence, absence and holding back all carry meaning, so a model of communication has to deal with what goes unsaid as much as with what's said.