Field not showing for business rule action to set field value 'same as'

[Josh Ferree]

I'm currently following this guide to simplify incident requests: Record Producer Dependent Fields (Choices) - ServiceNow Express Support

I've run into a snag with setting the business rule to map the new subcategory field to the actual one for incidents.   When choosing the action to set field value 'Subcategory' to same as the new field I've made for this 'IT Subcategory' I am able to choose Subcategory, but IT Subcategory is not showing up in the second dropdown.   I've attached a few screenshots of what I'm seeing.

[balaji_charapal]

As Amlan said if it is on Incident table only it will show.

You write advance script to set the values

current.u_subcategory = current.u_itsubcategory;