Cabotage occurs when a foreign carrier transports a customer between a U.S. origin and U.S. destination (including U.S. territories), regardless of the connecting point (U.S. or foreign).
As a general rule of thumb, when Delta is selling a ticket for travel that originates within the United States or a U.S. territory* and terminates within the United States or a U.S. territory, at least one of the segments must be on a U.S. carrier even if the connecting point is in a foreign country.
If travel originates from a U.S. city, Delta can not sell a ticket to Guam (GUM) with a connection in Seoul, Korea (ICN) if none of the flights are operated by a U.S. Carrier.
Delta can sell tickets to Guam (GUM) with a connection in Seoul, Korea (ICN) if one of the carriers is a U.S. carrier (no Codeshare flights).
*U.S. territories are Guam; San Juan, Puerto Rico; U.S. Virgin Islands; American Samoa; Federated States of Micronesia; and Midway Islands.