We have a case where a set of MARC tags use authorised values i.e. 902$a, 902$b, 902$c, 902$d. Each of these subfields will use different authorised values. We'd like the authorised values displayed for 902$b to change depending on how 902$a is set. For example, if 902$a is set to a specific value, then the irrelevant options for 902$b should be hidden so they can't be selected. This might go on i.e. the values for 902$c are then dependent on the setting of 902$b. These rules would need to be editable by the library.