When clicking the "Add New Upload" button you can choose between different upload types using the tabs at the top of the new upload window.
By default, "Upload by Email" is selected, in which case we will match activists in your upload to people on your list by email address, automatically deduplicating and adding new activists to your list if no match is found. See here for more information.
However, you can choose "Upload by Unique ID" instead to use the unique ID upload type. With this upload type instead of email you will provide the Action Network Unique ID as a column in your CSV. (You can download the unique ID of your activists using the reports tool, as a column available when you select the data you want to pull on a report. See here for more.) This upload type is useful for matching to other systems and not sharing files with the email address of your activists, for security purposes.
If you use this upload type, activists will be matched on unique ID (you will choose which column in your CSV is the unique ID in step 2 after adding the upload). If the unique ID in your spreadsheet matches an activist on your list (either subscribed, unsubscribed, bouncing, or spam complainted) then the upload will proceed to do whatever you chose for the upload to do (add tags, overwrite data, add new fields, unsubscribe, delete, resubscribe, etc...). However, if the ID you upload in the CSV is not found on your list already, the row will be skipped. Therefore, unique ID uploads cannot be used to add new activists to your list who aren't there currently (either subscribed, unsubscribed, bouncing, or spam complainted), unlike the upload by email type, which will add new email addresses to your list if no match is found.