1-That's it (informal)
A) used to say that sb is right
-That's it, carry on!
B) used to say that sth is finished
-That's it. The fire's out now.
C) used to say that you will not accept sth any longer
-That's it. I've had enough.
D) used to talk about the reason for sth
- So that's it-the fuse has gone.
2) That's that (informal)
used to say that your decision cannot be changed
- Well i'm not going,and that's that.
3) That's about all/it
used to say that you've finished telling sb about sth and there is nothing to add
- "Anything else?" "No, that's about it for now."