You need to set it up using conditional value lists. Do a search on this forum for "create conditional value list" to see an example.
Yeah, I have figured that out but the results I get from that search won't relate to -and thus, solve- my problem.
I would like some of the documents to belong to more than one section-category-subcategory. In order to achieve this, I created a second range of fields, tables and value lists that are clones of the original. But this is not so efficient
Right - you'd better use a separate join table for the individual assignments. See a basic example here: