Hufu 虎符 (Tiger Tallies) was a special two-piece object granted to officers, used for military authentification or authority. For instance, when a general wants to dispatch a troop from a certain, he probably has to show one piece of the tiger tally to the camp officer. The camp officer will in turn show his own piece of tiger tally and both pieces must match each other in order for command of dispatching/processing to go through.

Thus, tiger tallies work as a form of military authority authentification and security measures in ancient chinese military. The tiger tally can be seen in the chinese historical drama series Han Wu Da Di.

A good site about tiger tally is at
http://www.kongming.net/novel/tiger_tally/

I do know that tiger tally were used by officers. What about Emperors or High-rank Generals? What object was a symbol of their military power?

The Hufu is only one out of 4-level signet system. lower level ones are used to mobilize less critical military units. And contrary to TV-dramas, there wasn't only one Hufu but actually quite a number. Different Hufu were used to mobilizing different branches or garrisons from different areas. If I recall, Han Wudi kept a total of around 11 Hufus but this number actually fluctuates quite a bit depending on who's in charge.

Can i ask if Hu Fu is related to Ling Pai (令牌）or it is a totally different function all together?

Partially. But you have to remember that the Lingpai was more common in later dynasties.

Thing is, Lingpai was a general authority. Like of like how a police officer has the authority of stop and search. More commonly though, a Lingpai more of an identification in military or beuracratic installations. It takes the place of common paperwork. So if you carry a lingpai of a certain rank, you will have access to certain levels of passes and power to do a certain things within the chain of command. The Hufu on the other hand carries a very specifc order, like the warrent to search a particular house. It cannot be used twice and must be returned after the situation in which the order was needed has been resolved. Because it is issued by the emperor or someone similar, it's power to carry out that specifc order generally trumps other established regulations.

Of course, there are also specialized Lingpai that does the same job as a Hufu.

The full name of this is called the Tonghufu. Meaning 'bronze Hufu' because it is made of bronze. Another type was made of bamboo known as Zhushijie (竹使符).

The next level down are the Jie (節) was used when the emperor wished for an order to be carried out within the military. Be it altering battle plans or re-modification of the command structure.

The next level down is the Yuxi (羽檄) which was a single message or order that has urgency as it's piority and generally overides all other security procedures to get to its intended target.

The last one was the Xishu (璽書) which was just another message stamped with the imperial signate. As it is stamped, any disobedience were seen as instant treason.

Uses of Hufus in the Warring States is mentioned in chapter 77 of Shiji (the biography of the prince of Wei). In order to rescue Zhao, the prince stole the Hufu commanding the troops of Wei (from the king of Wei), showed it to their general, and took control of the army. I don't know of any other mention of this system in earlier texts, anyone?