A mixture. There are actually write-patterns that will
have a very high chance of identifying potential
bas sectors, significantly higher than just a conventional
surface scan. I think spinrite (-> google) was the first
to do this.
They cannot 'regenerate' anything. But the disk will do
automatic reallocation, when one of the special patterns
detects a problem.
All depends on whether they use patterns appropriate for the
low-level data coding. For MFM/RLL this was relatively easy.
Today they use things like PLRMLR (?) that are more complicated.
This could be a good product, but it is very hard to tell.