What XML tag is mandatory for calling APIs?

The <qdbapi> tag is the mandatory XML tag for calling Quickbase APIs because it serves as the root element needed for any XML requests made to Quickbase. This tag indicates to the Quickbase server that the incoming request is a valid API call and provides the necessary context for processing the request. The structure of the API call relies on this tag to understand the format and requirements of the subsequent data elements. Including this tag ensures that the server correctly recognizes the request as intended for the Quickbase data platform.

On the other hand, <ticket> is typically related to authentication and is used within the API request to validate the user's access but is not required for the structure of the XML call itself. Similarly, <rid> and <udata> are additional elements that might serve specific purposes in certain API interactions but are not mandatory for the structure of the API call at large. The presence of the <qdbapi> tag is what truly makes an XML-formatted request valid for Quickbase APIs.