It's pretty simple, actually.
If you make the new boot environment in the same ZFS root pool as the one you're copying from, it's just a ZFS snapshot and clone of the original pool.
So creating/destroying such boot environments is relatively quick.
That seems better at first. But it leaves the new boot environment tied to the one it was copied from.
On a long-lived server that gets regular updates, each time to a new boot environment, creating new boot environments in the same pool as the source boot environment builds up a nasty tangle of ZFS snapshots and clones that's extremely difficult if not impossible to manage. And because the snapshots/clones "pin" old copies of files, it also creates space problems over time.
Conversely, if you create the new boot environment in a different pool, it's a completely separate copy - files are literally copied from the source boot environment to the new one. It takes longer, and initially takes more space, but it's clean and totally separate.
In my experience, long-lived Solaris organizations tend to use the latter, often having Solaris servers with four boot drives - two hardware RAID-1 mirrors for two root pools. (Yes, HARDWARE mirrors. Replacing a disk in a ZFS mirror takes coordinated actions between the disk replacement tech and sysadmin. That coordination is hard to do when remotely managing hundreds if not thousands of servers in multiple locations. And as the replacement tech is from the hardware vendor doing warranty disk replacement, if the sysadmin isn't ready when the replacement tech shows up, he's likely to walk out. Replacing a disk in a hardware mirror is a simple "pull the disk with the orange light, pop in a replacement" action for just the tech doing the replacement. The admin gets to read the logs later, and there's no chance for fat-fingering a command.)
Just be careful when creating boot environments that you don't include users' home directories in the boot environment. Users tend not to like having their files rolled back to something they were at the moment you created the boot environment, and then rolled over to something else entirely when you have to switch to another boot environment for some reason.