There is no information about the CheckmarkBlock object in the XML output.
FineReader Engine does not currently support the field-level export scenario for the XML output.
Recognition results for a checkmark can be accessed via the corresponding CheckmarkBlock object and its properties CheckmarkState, IsSuspicious, and AmountOfBlack. The properties are available after a IFRDocument::Process or IFRDocument::Recognize method call.
It is possible to export the values of the CheckmarkBlock properties by implementing a custom export algorithm that will be able to write these values into an XML file.