Example for parent/child attribute

Create attribute Color&size.

1. Create parent attribute  

Create main attribute Color&Size without any values. Element type: - selectbox.

2. Then create new child attribute Size. Set Attribute Parent: Color&Size.  

Element type: - selectbox. Here you need add option values like XS, S, L...

3. Then create new child attribute Color. Set Attribute Parent: Color&Size.  

Here you need add option values like white, black, red..etc. Element type: - selectbox.

4. Add Option to Product  

Now if you go to product edit page click to add new Color&Size global attribute to editable product.

5. Add Option's values to Product  

Then add option value like black:XS black:S white:L white:XS red:S red:L or any other combination of product you have to sell.

In storefront customer can choose combined option value