A DataUrl variable is used to stored an external file, with many informations. Usually an image. It can then be used as a Text Variable to set a canvas property.
To use a Data-Url. Variable, use the text $myVariable, with the name corresponding to the field defined.
"canvasFields": [ {"type":"image", "src": "$portray", "left":5, "width":150, "top":40 ,"height":300} ]