TPA_Extensions is a generic container element for extensions to the OTA standard. It can be a descendant of many different objects, sometimes occurring at multiple levels within the same request/response.

If you came here from a page about a parent of TPA_Extensions, try going back to that page and looking at the TPA_Extensions field description. You should find links to its children.

You can also try studying the samples for the API call you're trying to make. The samples should give a full overview of the request and/or response. From there, you can search for details about children of TPA_Extensions.