To use as an identifier when importing and updating records from a separate system, an administrator should use External ID field type on an object. External ID fields allow storing unique identifiers from external systems and using them for matching records during import or update operations. External ID fields can also be used for upsert operations that insert new records or update existing ones based on external ID values. For example, an administrator can create an External ID field on Account object that stores account numbers from an external ERP system and use it for importing or updating accounts from that system. Rich Text field, Record ID, and Auto-Number field are not suitable for using as identifiers when importing and updating records from a separate system. Reference: https://help.salesforce.com/s/articleView?id=sf.fields_about_field_types.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.data_loader_upsert.htm&type=5

To create a many-to-many relationship between two existing custom objects, the administrator needs to create a junction object that has two master-detail relationships, one to each of the custom objects. This will allow each record of one object to be linked to multiple records from another object and vice versa. Reference: https://help.salesforce.com/s/articleView?id=sf.relationships_manytomany.htm&type=5