Parent Combobox feature with User Defined Fields

What is the Parent Combobox feature?

The Parent Combobox allows a user to define a parent child relationship between two separate Combobox Custom Fields. This will result when a value is selected from the Parent Combobox that the Child Combobox will populate with List Values that have been defined are related to the value selected in the parent. 

Can I create a Combobox relationship with a Project Insight default field (ex. Project Type)?

No, the Parent Comboxbox feature only works with custom fields (user defined fields) and only with the Input Control "Combobox".