How to Fetch Subform fields in Multi Select Lookup?

How to Fetch Subform fields in Multi Select Lookup?

Hello.

How can I lookup a field of subform into another form with multi select ?

I have a form where I'm adding skills and subset of skills along with the value of every skill.
In the main form, I want to get a calculated the value on the basis of skills I selected.
I used a lookup field to select skills, which is working
But when I'm using lookup field to select subset of skills, it's not showing.
What can be the alternate method to display subset of skills in the field?
For example: if selected_skill = SEO
then fetch all skills related to SEO into the field to select and then calculate the value accordingly.




This is the form, I selected SEO from the lookup field, and In place of choice 1 2 and 3 I want to show all subskills added with SEO in the New Skills form.

Image is attached below as well. 

Please suggest.
Thank you