They took an unauthorized short-cut. Not in the book;
"Q: How do I fasten vinyl siding to a wall?
A: Choose aluminum, galvanized steel, or other corrosion-resistant nails, staples, or screws. Aluminum trim pieces require aluminum or stainless steel fasteners. Remember, as temperatures change, vinyl siding can expand and contract ½" (12.7mm) or more over a 12'6" (3.81m) length. " From; http://www.vinylsiding.org/installation/faqs/
It appears to be a full width (double board) panel, not per fig. 48.
Per instructions, you can use it to draw the roof slope pattern, fig.45 and fig. 61, but only apply it horizontally or vertically, also; it appears to be missing the kick-out flashing required for quality installation; fig. 44; I'd be more worried about that. Hopefully the WRB was installed properly to shed water if the full length piece contracts the 1/2" all at the bottom J/ depending on fastening, though the slots would restrict it to less. Download the installation manual (in Blue color type).