Excuse me, what does "the total number of IPC4 categories to which patents belong" mean?

This should also be translated in the context, generally translated as "the total number of four categories in the IPC classification to which the patent belongs" or translated as "the total number is in the four categories of IPC to which the patent belongs". Among them, class refers to the "big class" in IPC. Ipc (international patent classification) international patent classification system establishes categories according to technical topics, and divides the whole technical field into five different levels: section, category, subcategory, main group and subgroup.