APS Provisioning Architecture
The following picture demonstrates basic APS application provisioning scheme including hosts involved into the provisioning process and data flow between them:
In case of provisioning APS application of the Shared Web Server provisioning type POA directly communicates with the target Shared Hosting server where an application will be installed.
In case of provisioning APS application of the External System provisioning type POA works through a Provisioning Gateway Host. Also, in this case additional gateway may be involved, like in case with Office 365 or Hyper-V APS applications.
See the main KB article #115664 APS: General information, Best Practices and Troubleshooting for more information.