To answer your question I am going to use the words of the US patent office’s.

Below is the intro for the USPTO  seven steep process to conduct a patent search using there system.

This is a Seven Step Strategy for conducting preliminary searches of U.S. patents and
PTRCs provide online access to the PatFT (Patents Full-Text and Image)
and AppFT (Applications Full-Text and Image) databases on the USPTO website
 and the Espacenet Worldwide Database on the EPO website.
In addition, most PTRCs provide access to additional USPTO patent databases
available through PubWEST (Public version of the Web-based Examiners Search Tool)
and PubEAST (Public version of the Examiners Assisted Search Tool).
The USPTO has transitioned from 100 year old U.S. Patent Classification (USPC) system
 to Cooperative Patent Classification (CPC),).
The USPTO currently only uses CPC for classifying new utility patent documents;
it will continue to use U.S. Patent Classification for classifying design and plant patents.

Let me tell you, it only gets more convoluted from there. Do you see the problem?