The different files expand to different image sizes for various sized flash media, 1 Gigabyte, 2Gigabytes etc.
There is very little advantage to using a larger image even if you have a large flash drive. Manufacturers often label a flash drive '2GB' when it's actually something slightly smaller leading to the image failing to write.
Using a 1GB image on it ensures it will fit.
It also writes a lot faster, an issue for me since my CF card reader is only USB 1!
I was trying to find a post showing what will likely happen when you boot the usb drive but failed.
Assuming that your system is able to boot from the USB drive without issue it will very liekly fail to a command prompt complaining it can't mount root. This is because, as I said, it expects to mount root from an IDE connected device. It should then give you a list of possible locations and you can type one in.
Trying to mount root from ufs:/dev/ufs/pfsense0
ROOT MOUNT ERROR:
If you have invalid mount options, reboot, and first try the following from
the loader prompt:
and then remove invalid mount options from /etc/fstab.
Manual root filesystem specification:
<fstype>:<device> Mount <device> using filesystem <fstype>
This is equivalent to: mount -t cd9660 /dev/ad0 /
? List valid disk boot devices
<empty line> Abort manual input
At that prompt you probably want to type: ufs:/dev/da0s1a
However I'm not 100% on that!
Once it has booted correctly you will have to edit the fstab so that it knows where to mount root from in the future.