Optional model that becomes available through the '_in.*' script variables. The format is attribute:value;attribute:value...
Alternatively, if the model is large, it can instead be included as a Json object at the top of the body. To do this, set this attribute to the special word 'body' (no quotes),
then include a Json model object at the very top of the body. The last curly bracket needs to be on a line of it's own (no spaces) to mark the end of the model and the beginning of the actual body expression.