Do mean that a given sub category value can be a member of more that one category?
If so, two different approaches may be used.
One is to list each subcategory again with a different category value in the category field.
The other is to set up the category field as a check box field of all your categories.
Both approaches can work and arguments can go either way as to which approach is best.
Either way, you set up this relaitonship:
DataEntryTable::Category = SubCategories::Category
For 10 different ways to set up a conditional value list, see: