PayPal API TransactionSearch via PHP

I am working on using PayPal's API for TransactionSearch - https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_r_TransactionSearch. The sample php code is at https://cms.paypal.com/cms_content/US/en_US/files/developer/nvp_TransactionSearch_php.txt

I have a PayPal business account, and I have created an API username, password and signature. Using the same PayPal account, I granted access to my API username so that I could begin testing. Using PHP and (cURL), I have successfully sent and received information via the API. All works fine.

The goal is that I will provide my API username to multipl开发者_C百科e end users, so that my application can query their PayPal information, and import their historical records into the application for the users to view. The part that I dont get: using the TransactionSearch API, I dont see anywhere to specify a PayPal Email address - i.e. the PayPal account that I am going to query for historical information. How or where do I query multiple individual PayPal accounts?

It seems that there is an additional API parameter that can be used called SUBJECT

SUBJECT Optional Email address of a PayPal account that has granted you permission to make this call. Set this parameter only if you are calling an API on a different user’s behalf.

https://www.paypalobjects.com/en_US/ebook/PP_NVPAPI_DeveloperGuide/overview.html#2076088