Installing Windows 10 via Boot Camp & official Microsoft retail USB TL:DR version.
If I bought an official retail USB key of Windows 10, can I use it as-is in Boot Camp Assistant or do I need to make a new key from a downloaded .iso?

A question on Super User prompted me to try research this, but Apple's Boot Camp instructions are somewhat confusing...
From Apple KB : How to install Windows using Boot Camp
emphasis mine.

2. Get a Windows disk image
When you purchase Windows, it comes as a disk image file (ISO) that
you download, or as an installation disc or USB flash drive. If your
copy of Windows came on a DVD, you might need to create a disk image
of it to work with Boot Camp. If your version of Windows came on a USB
flash drive, you can download an ISO from Microsoft.

OK, deep breath...
Does that actually mean that if someone went to the trouble of going to a real store &amp; buying a real copy of Windows 10, in a box, with a USB installer &amp; serial number on a card - that all they really gained was a serial number on a card &amp; they still have to go through the process of making their own USB stick from a downloaded .iso?

If you follow Apple's KB trail to try figure this out, this is what happens...
You start from Apple KB : How to install Windows using Boot Camp &amp; click the link at


Use Boot Camp Assistant to install a new copy of Windows.


which takes you to How to install Windows using Boot Camp, my opening gambit from above &amp; the link

you might need to create a disk image

which takes you to Create an ISO image for Boot Camp from Windows installation media
the second paragraph of which states...

If your copy of Windows came on a USB flash drive, see Use Windows 10 on your Mac with Boot Camp for more information.

...which takes you right back to where you started, ready to go round again...
I believe that you will need an actual .iso image of the version of Windows you have purchased retail. Fortunately that is both easy and legal, just go here.

Download the same version you bought and save it to your Mac's hard drive. I would use a different USB key so you don't lose your original. But then if all you need it for is Boot Camp on the Mac go for it and use your Windows USB key with Boot Camp.