For example, if you are inserting Account records, you could specify a CSV file called insertaccounts.csv containing a Name column for the names of the new accounts. Click Next. After the object and CSV file are initialized, click OK. If you are performing an upsert, your CSV file must contain a column of ID values for matching against existing records. The column is either an external ID (a custom field with the External ID attribute) or ID (the Salesforce record ID).
What data loader do you use? I prefer to deal with this reliable salesforce data loader https://skyvia.com/data-integration/salesforce-data-loader. You see, I believe you can have a look at this tool I used to work with. You see, it is a powerful Salesforce integration and data loading solution that offers a number of advantages over other Salesforce data loaders. Hope it can help
For example, if you are inserting Account records, you could specify a CSV file called insertaccounts.csv containing a Name column for the names of the new accounts. Click Next. After the object and CSV file are initialized, click OK. If you are performing an upsert, your CSV file must contain a column of ID values for matching against existing records. The column is either an external ID (a custom field with the External ID attribute) or ID (the Salesforce record ID).
What data loader do you use? I prefer to deal with this reliable salesforce data loader https://skyvia.com/data-integration/salesforce-data-loader. You see, I believe you can have a look at this tool I used to work with. You see, it is a powerful Salesforce integration and data loading solution that offers a number of advantages over other Salesforce data loaders. Hope it can help