That’s it! vs That’s that!

[Tamara]

Hi

That’s it!
That’s that (then)!

As I understand, both the conversational expressions are used in conversation to say 'OK, now we’ve finished, that is the last thing and that is agreed'.
Right?

Could you explain the difference, if any?

Tamara
_________________
It’s impossible to learn swimming without entering the water…

[Alan]

Hi Tamara,

That's it could be: That's just what I was looking for - you've found something in a shop you've been looking for.

or it could be the prelude to an outburst: That's it - I've had enough and I'm going home. Good bye!

or it could be: I think we've finished now.

That's that ( plus often: and all about it) suggests a conclusion said with an air of resignation because you cannot proceed any further as in: You have applied for a mortgage and nobody offers you one and you realise there's no hope of getting one and so your comment is: Well, that's that then.

Alan
_________________
English as a Second Language
You can read my ESL story A New Season

[Tamara]

Hi,