system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] How are different types of training in WH40k comparable to American military training.

I'm aware of how elite the Space Marines are, but how elite? Could it be compared to SF selection? What about an Ecclesiarchal Confessor? An Adeptus Arbite? An Imperial Guardsman? Are they on par with a modern infantryman, beyond, etc.
gpt
Standard military would be the PDF regiments, whose quality vary wildly depending on the world, or even part of world, that they hail from. Whatever immediately comes to mind when you think of "The Army", that is the PDF.

Imperial Guardsman are fairly elite, despite what you'd think regarding their numbers and casualty statistics. What actually constitutes their training depends on the world they come from and the regiment they are part of but generally it's pretty difficult and it's a lot more standardized than the PDF. Easily the equivalent of joining the American Marine Corps, or some other Earth equivalent. Special Forces would be more akin to something like the Karskin, or some of the other elite scout and shock units that regiments employ.

The Arbites are basically cops from hell; they receive more or less the same training as a Guardsman but most have a considerable amount of prior experience in law enforcement or military prior to becoming an Arbite.