Before an incoming packet is passed to the proper operator, it is validated by Plesk (by the Agent Engine module). The module retrieves the XML API version from the packet header, chooses the matching set of XML schemas, and validates the packet against the given schema.
If the packet is considered invalid, Agent Engine forms an error report, wraps it into a response packet, and sends it back to the client. If all elements of the packet are validated successfully, Agent Engine checks whether it sends the packet for execution.
Plesk parses each operator-related section, forms calls to internal functions, and returns the result of the executed commands to Agent Engine.