Using Coupa as an example, the Domain, Identity, Sender Domain and Sender Identity values want to be the DUNS number for your business. The descriptions of the various fields can be seen on their documentation at PunchOut Site Settings
A DUNS is a unique 9-digit identifier for businesses. Your business may or may not have a DUNS number. A DUNS number can be requested, checked, or updated through Dun & Bradstreets
site. You do not necessarily have to get a valid DUNS value in order to complete the CXML PO integration for your site immediately, you can just enter "DUNS" for the Domain, Identity, Sender Domain and Sender Identity fields.
Any behavior or use of these values in a CXML PunchOut integration could change depending on the procurement system you are using. Please check with support for your procurement system if you have further questions on the use of these values and the relevance of a DUNS number to your transactions.