How can a Twitter tweet be used as legal evidence?

1) Twitter keeps copies, which include originating IP address, logged in person, timestamp, etc.

2) You would subpoena them for the log.

What’s your use case?

General curiosity, but I also wondered if Trump could be held to what he says on Twitter

Yes, a tweet can be used to demonstrate one’s state of mind at a given time. It is the same as any off the cuff remark.

Trump cannot be held to what he says on twitter anymore than you can hold him to what he says everyday from the podium.

I understand what you are saying, but it seems that he can’t be held to account for tweets any less than for what he says on the podium, for what that is worth.