Ex: if i have a custom field named "Department" (with value Dept 1, Dept 2) and one more custom field named "Users" (with values User 1, User 2, User 3, User 4).
When i select "Dept 1" in "Department" custom field, then "Users" custom field should display User 1 and User 2
When i select "Dept 2" in "Department" custom field, then "Users" custom field should display User 3 and User 4