Where is Communication preferences or Channel in service request maintained in customer form in AR?

Where is  Communication preferences or Channel in service request maintained in customer form in AR?

So that it will defaulte on the service request transaction if added in AR.

If this not is possible, which other fields can be used?

Business Requirements:
Need to tag customer in AR that invoice from service will be sent on e-mail or electronic format. Not sure which field to use that also will update already created service requests