OK, Dan, i'll try to simplificate
Can't be bothered to giggle it all, so you'll have to accept my fragile memory, or giggle it yourself.
Up until Mr. Whitworth, every user made their own thread, to their own profile,with their own head-size.
Whitworth laid out a table ofdiameters thread form, threads per inch and ratio of head-size to shank diameter.
Along came the finer-pitch BSF which had a smaller head in relation to the shank, but these were the same sizes as Whit. heads,except the shank was thicker....hence a1/4 W spanner fitted a 5/16 BSF fastener.
THE SIZE DESIGNATED, WAS THE SHANK DIAMETER
The American AF system (Across Flats ) designated the head size...an open-ended 1/2 AF spanner is 1/2 inch nominal between the jaws. there is, again, a direct correlation between head and shank size, unlike the old British standards, it is a constant, irrespective of thread pitch....a 1/2 AF spanner will fit (iirc) a 5/16 unified fastener, wether it be coarse (unc) or fine (unf)
metric system ...broadly similar to the unified system in that a mechanic will often refer to "a 13-mil bolt" but means one with a 13mm AF head
All threads, with the exception of Metric, have a consistent pitch/diameter ratio....for a given type (say, UNC,) the threads per inch-length (pitch) will lessen as the diameter increases.
broadly, that applies with Metric, but there are a couple of "oddballs" where the pitch is as expected, but also, the same diameter can have a pitch associated with the next size-step.
As I said....the continual evolution is good business for the tool manufacturers.
"philips" screws???...are you
sure it's not a Reed and Prince (very common on Jap consumer-goods) or a Pozidrive? they're allcross-point but there the similarity ends A philips driver will bodge a poz. screw in, but a poz. driver won't seat in a Philips head....neither will properly do a R&P head!
All, of course, claim their advantages. Fasteners are an engineering subject in their own right, i've only scratched the surface here!