Why is one not able to select a different Party/Account?

Hello,

Navigator --> Depot Repair Super User --> Service Request --> Service Requests --> Create Service Requests

Where:  Under the Contacts/Addresses tab

What:  Bill To and Ship To Party/Account

Questions:

Why does the Party field under the Bill To Address and Ship To Address not allow me to select a different Party than the Party of the Service Request header?

For Example--How would a user of this form do the following:

1) Item is installed at Party A (which is in the Service Request header)

2) Bill To Address is a Party A address

3) Ship To Address is a Party B address