Hi Vlasta,
I am not sure how these avada fields are stored. You have multiple good viable options:
Custom fields
It might be more beneficial to create a new custom field – perhaps using ACF – instead of using existing ones.
If a company can have more than one NACE code assigned, then using ACF is probably a good idea, as it supports checkbox/multiselect fields to allow entering more than one value – that the search can later use within a filter.
Taxonomy Terms
This is another good option, it might save some database space as well. Taxonomies can be created via 3rd party plugins, or using custom codes, depending on the use case.
The main advantage over custom fields is, that each taxonomy term is a separate object, having a separate ID and label. In this case, for exaple if multiple companies will use the same NACE codes, then it does not have to be stored multiple times within the database, saving some space. It is the same thing as post categories, where a category can be assigned to multiple posts.
If you are planning to use the NACE codes as search filters, then the taxonomy terms solution is much better.