Hi there, @royjay… you can go one of two ways here. You can add an attribute to the sub-category option set that links each sub-category to an option in the category option set or you can add an attribute to the category option set that defines a list of sub-categories that are associated with each category.
Regardless of which way you go, your choices source for the sub-category dropdown will be all of the sub-category options filtered to show only the options that are associated with the selected category.
Hope this helps.
Best…
Mike
Edit: if a visual would help, here is the dropdown for the sub-categories, and it is based on the sub-category option set having an attribute (called category) that links to the category option set, with each sub-category being associated with an option from the category set.
