Trunk group configurations are a combination of two variable sets
termination/origination
vendor/customer
Termination - Vendor
Traffic originates with you and ends at the vendor.
Termination - Customer
Traffic originates with you and ends at the customer.
Origination - Vendor
Traffic originates at the vendor and terminates at you.
Origination - Customer
You configure traffic that originates at a vendor and terminates at the customer.