Unable to create project descriptive flexfields with binding variables

Summary:

We have a business need to display conditional descriptive FF’s when a project type is “capital”. When a capital project is created we would like an extra DFF to appear in the user interface so the user can select an appropriate asset type.

So I like to pass a project_id as a binding variable in an sql which will result in a project type. If the project type is “Capital” then Asset Type must be displayed. However when saving the dff definition I receive the error, which seems to suggest binding variables are not supported in this projects creation user interface.