AFAIK there is no official documentation available. If I remember correctly than there are a short chapter in two QlikView books which describe some of the advance transforming features which are available within the QlikView load-wizard (I don't know if it's available in the Sense wizards, too).
In your case it means that there is a string-condition applied which replaced NULL's but I'm not sure if they are replaced with 1 or top respectively if it will be applied for the first row or column ... If you look within the above mentioned books you will get some hints about order and meaning of the parameter. Also you could use the above wizard to check it out with some reverse engineering approaches.
But honestly I wouldn't do it because there is nothing which couldn't be reached with "normal" where-clauses or other transforming steps.