Is Rick's packing list of shirts, pants, underwear and socks, limited to clothes actually packed or is it meant to include clothing one is wearing?
It's just a guideline. However, I always include what I wear on the plane in my own packing lists.
The clothes you wear on the plane should be able to be mixed in with everything else to make your travel outfits. So no wearing something on the plane if you can't wear and wash it with your packed clothes. This is especially relevant when it comes to shoes. PACKING 2 pairs of shoes might be difficult if you have larger feet or need heavy shoes, but packing 1 and wearing 1 is much easier. So your "plane shoes" need to be one of the pairs you will wear a lot on the trip.
The list includes what one is wearing.
There's a cute bit in one of his books where he claims to strip naked and take a picture of everything he packs. The cutline lists everything in the photo and ends " ... and one camera, not pictured."
To me, any packing list whether it's Rick's or someone else's, includes the set of clothing you are wearing in transit. So if it says 4 pr pants or bottoms then pack 3, wear 1.
If you'll look thru some of the posts on here you'll also see that many of us pack less than what Rick has on his guideline list.
Thanks everybody - very helpful.
Per Rick's article, "Here’s a rundown of what should go in your suitcase:" It's a good starting point for those questioning themselves about what to take. I pack lighter than the list and specifically deviate by excluding convertible pants (UGLY) and do not pack five sets of socks and underwear. Merino wool socks and ultrafine merino wool underwear are also in my bag.