"Suppression list" refers to a text file of email addresses that can be excluded from receiving an email campaign. When the campaign is sent, the contacts in the segments/categories assigned to the campaign will be scrubbed against the suppression file assigned to the campaign.
Typically suppression lists are manually uploaded to the Contacts dashboard and assigned at time of Campaign creation. Currently there is not a way to create a suppression list via API or FTP upload.
Please reach out to our Client Success team to discuss your needs if you require the ability to create and assign suppression lists via API or FTP. A custom solution may be possible.