There is a custom field type that's called: "Drop Down List (User)". What is this type? What users will it include?
This custom field type will list users in a drop-down. The list of users will have their name as well as their thumbnail profile image to left of their name.
The users that will be available for selection are the admins of the project and the list of team members of this project. Team Members are users who have minimum of editor permission within that project.