I'm sorry you attached your question to this very old topic, because almost all of the information in the original post is now out of date, and people reading it may not notice that it was posted in 2009.

To answer your questions:

The BIT ticket can be used on any bus or tram, or combinations of these, for 100 minutes. It can also include one entry into the metro system; this means that you can change from one train to another at a station that has two trains (for instance Roma Termini) but you can't exit the metro system and re-enter on the same ticket.

For example, you could take a bus to a metro station, then take a metro, and when you exit the metro station, you could take another bus. Or you could take a bus to see a museum and then, if there was still time left on your ticket, you could get back on the sme bus, going in either direction.

A few weeks ago, we were in Rome. We took a bus to St. Peter's Square to see the tree and the nativity scene, and to meet my daughter there. After our visit to the square, we took the metro back to our hotel. The ticket had only five minutes left, but we were able to use it to enter the metro system.

As for the BIG ticket, you should be able to use it up until the time the ticket expires. You can validate it every time you use a bus, but I don't think it's required. It used to be that you couldn't do this, as it put a printed stamp on the ticket which overwrote the original time you used the ticket, and made it impossible to see how much time was left. Now there is a magnetic stripe, so it doesn't matter. If you put the ticket in the machine, the display will show how much time you have left.

Both tickets are also valid on urban trains and regular trains within the Rome city limits. On the metro/train map, the last station for which the ticket is valid is printed in red letters.

http://www.atac.roma.it/files/doc.asp?r=4

Hi bvlenci,
Thank you so much for your answer. Yes I know this is old article, but I just hope I will get answers for my questions above. This will be my first visit, so the transportation system there is new for me, in everything.
I want to make sure that BIG ticket is really time basis, and can be valid really for 24 hours. (so it's still useful if I validate this ticket for the first time in the night, like about 10 pm). So BIT and BIG ticket are really the same functionally, and the difference is just BIT is valid for 100 minutes, and BIG is valid for 24 hours.
Yes that's useful if there are magnetic stripe machines there now to check the left time in the pass. I hope there are such machines inside the buses or at the bus stops, so I may know for sure that I'm not against the law to use the pass to get on the bus. (as I just can validate once for the pass)