The US Patent Office assigned these classification codes to US patent publications to organize their own search files.
With the advent of the newer CPC system, these codes are no longer assigned after 2015. They are still searchable, however, going backwards in time.
In the drop-down menu, you can choose only the main code or both types of code. US classification codes are revised every two months and retroactively applied.
These codes can be searched at the main class (3 digits) subclass (6 digits), the full decimal code (9 digits) or the complete code with extension (9 digits plus 1 to 3 alphabetic characters) levels.
Main class:
108
Subclass: 108051
Decimal: 108051110
Extension:
188250000B