'Set field values' drop down list does not show expected fields for table when creating a new Business Rule
Understanding Business Rules security in 'Set field values'
If you have an issue where you are creating a new Business Rule for a particular table, and the drop down list for the 'Set field values' field does not show one or more columns present in that table, the likely reason is that the 'save_as_template' access control rules active for that table, or a parent table in its hierarchy, do not allow rights to set the value of that particular field.
For example, the out of the box access control for the 'save_as_template' operation for incident.* has the script field set to false. If you were using a business rule to set fields in the incident table using the 'Set field values' field, and did not see the expected fields, you could either add a new save_as_template access control for your particular field, or adjust the existing access control.
If you have 'Admin overrides' checked on the access control and you are an admin, but still do not see the expected fields, it is likely that a different access control without the 'Admin overrides' checked was evaluated in the process of checking privileges. In such a scenario, the instance security will then go back and ignore Admin overrides on any other ACLs that were evaluated. This is by design for security purposes. Ensuring all ACLs that are evaluated have admin overrides checked will keep this from happening.