Which circumstances will prevent a system administrator from deleting a custom field?

A system administrator cannot delete a custom field if it is part of a field dependency. Field dependencies create a relationship between fields, where one field's available values depend on the selection made in another field. If a custom field is integral to such a dependency, its deletion would disrupt this relationship, causing potential issues with data integrity and user experience. Therefore, the system enforces restrictions to prevent deletion in this case, preserving the structure and functionality of dependent fields.

While a field being used in a workflow field update, a report, or a page layout might indicate its importance, these scenarios do not inherently prevent the deletion of a field. For example, while removing a field from a page layout will not delete the field itself, the dependency of the field on another is critical for maintaining valid relationships and functionalities within the system. This necessity makes it crucial for the system to protect dependent fields from deletion.