Flowchart reference number 7.
Presidential signing statements are the least valuable to collect for a legislative history because they only happen after Congress has acted.
Read this article by Laura McDonald, Florida State University Law Review, Vol. 38, Issue 1 (Fall 2010), pp. 179-208; 38 Fla. St. U. L. Rev. 179 (2010-2011):
Interpretive Worth of Presidential Signing Statements: A New Form of Legislative History (Heinonline)