My Use Case:
List 1 stores people.
A person belongs to one business unit. (Dropdown list)
List 2 stores projects.
A project belongs to one business unit (Dropdown list)
List 3 stores knowledgebase articles.
A knowledgebase article belongs to a project (Dropdown list)
When I pick a project for my knowledgebase article it will look at the business unit the project relates to and secure the knowledgebase article to everyone in that business unit.
The problem arises when I am on a project that is not from my business unit. i won't be able to see projects that are not from business unit in the dropdown list. The project items are secured to people in the business unit the project relates to.
Therefore I want to have an option on the person list to add one or more projects where the projects are not from my business unit.
Hence the need to present a multi-select dropdown of all the projects that are NOT in my business unit.
I hope this scenario makes sense.