References allow you to display images and place them anywhere over the canvas.
Tap on the reference icon in the top bar to open a reference window. A second tap hides all reference windows. 
Tap on the import icon to choose your image from the Photo Album, your Files, or directly shoot a photo with your iPad. 
Rotates the image 90° clockwise. 
Fits the image to the size of the window. 
Adds a new reference window. 
Deletes current reference window. 
Drag the corner arrow to resize the reference window.
The reference icon allows you to hide or display the existing reference windows without deleting them. You can use 2 fingers to pinch and zoom in and out directly in the images.
