When using the Advanced Search, there are a series of helpers that make searching using classification codes much easier. These are useful since they help you find the classification codes that are used for specific technology fields.
You can find the Classification Code Helpers next to some of the classification code search options at the end of a specific field:
You will notice that for both "Main IPC" and "IPC" (International Patent Classification), the helper option is "IPC Helper". These are not the same since the helper next to "Main IPC" will search for MIPCs and the helper next to "International Classification" will search for IPCs.
To use the Classification Code Helpers click the button of the search helper for the classification code you are interested in using. From here, type into the search bar for this helper and click the "Search" button. You will then be given a list of options of, in this case, IPCs that use this input. Click on the green + icons next to these to add them as refinements for your search.