Porch construction: orientation of flooring boards (perpendicular or parallel to siding)
That's a &quot;breadboard end&quot;, and it's simply a design element.
There is no requirement for it that I'm aware of. If you do an internet search for deck images, you'll see a wide variety of decks with and without this type of breadboard edging, and the ones with it will have it in varying widths. It's purely aesthetic.
The most important thing is to have metal flashing that runs up under the last piece of siding above the deck and across the top of that tripled ledger board. The metal will help shunt water off the top of the ledger instead of letting it sit there and rot it out.