List fields (Selectbox, radio button, checkbox, multi selectbox and open list) all have the same "specific property": the bind_type, which represents "what you can select in this list". It can have three values: static for fixed values (for example the "Severity" list field in the requests tracker), users when the list allows selecting users (for example the "Assigned to" list field in the requests tracker) and ugroups when the list allows selecting groups of users.

This information is stored in a common table in the database: tracker_field_list. However, only in the case of multi selectbox and open list fields, when the tracker admin deletes the field for good (removed from "unused list"), the bind type is NOT deleted and stays in the database, forever.

It should be removed at the same time as the other specific properties of the open list and multi selectbox fields.