Construction contracting is a construction company (generally known as party B) through the bidding process to obtain the project is called contracting, there is the meaning of building, carrying in it.

The contracting is generally the owner of the unit (generally known as party A, that is, the money, or the construction side) issued bidding documents, selecting the construction of the construction unit is called the process of contracting.

The above is my personal understanding! You can also search for "contracting" and "contracting" in Baidu Encyclopedia, to understand.