Understanding a B2B scenario

Dear Experts,

I have limited knowledge of XI particularly with regard to config and development.

I started looking on one scenario in this link which is EDI related

http://scn.sap.com/community/pi-and-soa-middleware/blog/2013/07/24/quick-overview--b2b-scenario-with-seeburger-edi-adapter-using-integrated-configuration

If someone can go through this scenario,my question is :

Who are following parties exactly ?

EDI_SP_MEX_AS2_PROVIDER        : I conclude the business system from where we recieve PO file and one to which we send MDN back.

EDI_SP_PI_AS2HUB                       : SAP PI

EDI_BP_MEX_ELECTRONIC           : ERP System of mex electronic

But i am not sure ,may be i am not understanding this well.

Regards.

Aditya Sharma