Partitioning usb disk drive for multiboot

[Dave Feustel]

I would like to partition a usb disk drive with enough partitions to
boot 4 different linux/freebsd operating systems bootable using grub.
Unfortunately, (among other things) I am confused by the differences
between partitions and slices (which I first read about in the context
of OpenBSD). Do I need a /boot partition for each OS or just one /boot
partition with images of all the different kernels I want to boot?

Thanks.