what happens to existing partitions during install?

[Andras Simon]

If I have a number of partitions from a previous Fedora version,
and chose custom layout during installation of F12 (just to be
able to continue to use the same partitions as before) what
happens to those partitions? Are those that are needed for the
installation (/boot, / etc.) formatted and the rest (notably
/home, /usr/local) left alone? The installer warns that

"Any data on deleted or reformatted partitions will be lost"

The "deleted" part is clear; but what are the reformatted partitions?
I see no option to mark a partition to be reformatted.

I've read the relevant part of
http://docs.fedoraproject.org/install-guide/f12/en-US/html/ but
didn't find an answer to this. I'm pretty sure that "what can be
left alone is left alone" but it would be reassuring to see a
note to this effect either in the installer of at least the
installation guide.

Sorry if I'm missing something very obvious!

Andras