1. She couldn't have seen him yesterday.
Impossibility (past)
He's on holiday in Malta.
2. I couldn't open the door.
Inability
3. Could you say that again?
Request
4. We could either go to the pub or we could get a take-away.
Possibility
Are there specific phrases to name a communicative function?
They aren't set in stone as absolute terms, but generally people tend to use the same words or similar ones.
