Not sure who you are quoting there. But to me, a clean install involves erasing the disk and installing the standard version of the operating system, making your computer behave like a new one the next time you start it up.
I assume that regular install means that you will leave all your existing files in place and let the software installer just replace the necessary files to upgrade. In years past, people suggested the clean install to minimize problems with leftovers from old software. In my experience, that is no longer necessary (unless you're trying to address some specific issue).
I did the regular install of Sierra on my MacBook Air because to continue using some expensive legacy software.
On my Mac Mini I did a clean Sierra install to an external SSD because I am migrating to new software and want to keep it "lean and mean". However, I left Mountain Lion on the internal SSD to I can boot into it when needed.
If you want to continue using old software, the regular install makes it much easier, however I've also had pretty good luck using Migration Assistant in the past. If you try start from scratch and run the original installers from old software however, then you may have some problems.