Assume the following:
You create a new contact (let's name them "Person A"), and add two separate numbers for them - a "Home" and a "Work" phone number
You then send them a message to their Home number from within Messages.app, which creates a new conversation thread
Person A then sends a message back, but using their Work number
In this situation, when you next reply to their message, which number will Messages use - will it use their Home number (because that was the number that originally started the thread), or will it use their Work number - because that's the number that they last used in the conversation?