The Upload screen is used to either upload a CSV or Excel file manually, or to choose where the External Data should be pulled from.
On this page you can upload a CSV or Excel file, depending on the source selected on the previous screen. You will also select the Target Salesforce Object that you want the data to be imported into.
If you have recently uploaded a file already for this job, then you will have the option to proceed with the recently uploaded file, or to upload a new file:
For an Export Job, you will select the Salesforce Object that you wish to Query:
Here you will have the option to Append to current data. Selecting this, will present you with a File Upload. You can then append the query data, to your existing CSV file (like a VLOOKUP in Excel).