At a quick guess I would guess that sig stands for significant figures
-- not sure that exact meaning of 3 PLUS 13 but it looks like the old CAPA language. http://thep03.baylor.edu/CAPA/digits.html
were these in problems translated from the CAPA language? (these tended to be problems on physics.) could you post a path to an example? This is not a common usage for webwork problems but it looks like something that might have been provided for backward compatibility.