Summary
The attribute 'update_synch_custom' is for internal use only by ServiceNow development. It is supported by a back end Java class, part of embedded code, not accessible by customers. This is by design.
Steps:
1. Login to the instance <abc>
2. Navigate to sys_dictionary.LIST
3. Add 'Attributes' field using gear icon
4. Search for 'update_synch_custom' in Attributes
