The difference is the type of label that is printed and if any additional forms are necessary, like Customs Declarations, Commercial Invoices, or Certificates of Origin.
ShipStation automatically adds customs declarations to orders with international shipping addresses. By default, the information added to the customs declarations is based on the product information included in each order.
Customs Forms & Commercial Invoices Do Not Auto-Print
For carriers that do return forms that require printing, these forms will not automatically print with the label. You will need to select the forms to print separately from the label printing step.