Apart from standard attributes like a name or manufacturer, parts can also have custom fields/attributes. Like the "Notes" field, these can be used to store any data, but in a more structured manner. Some usage examples include: Distributor, Distributor Part Number, Supplier Part URL, Weight or Container Tare Weight.
This allows for extending the standard set of part attributes with company-specific data, in a structured way.
Custom fields are indexed for searching, so they can be used to find parts.