Below is a copypaste of two e-mails from docs.org explaining the issue.
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Mail 1 from Adam Williamson:

Someone on the forums kindly pointed out an inaccuracy in the
Installation Guide:

https://docs.fedoraproject.org/en-US/Fedora/19/html/Installation_Quick_Start_Guide/s1-diskpartsetup-x86.html

"All data will be erased during installation from the disks you select
at this stage. Do not select disks that contain data you want to
preserve."

This is not correct. It would be correct to say that no data can
*possibly* be deleted from any disks you *don't* select at this stage,
but it is not true that all data will be removed from disks you *do*
select. Suggesting that this is the case prevents people from following
lots of common install paths.

What actually happens to the data on the selected disks depends on the
user's choices at Installation Options and 'custom partitioning' or
Reclaim Space. If you go to custom partitioning, basically, you're in
complete control of what happens; you can do all sorts of stuff from
there, but no data/partitions will be lost unless you explicitly choose
for that to happen.

If the disk has sufficient unpartitioned space and you just pick the
easiest 'install into empty space' path, no existing data will be
disturbed, though any existing bootloader in the MBR will be overwritten
(this is normal and has always been the case).

If you go through Reclaim Space, you get the choice of what existing
partitions to delete or resize.
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Mail 2 from Chris Murphy:

Yeah, the user actually needs to go to some extreme lengths to erase all data on the selected disk. In Custom, it's deleted one partition at a time, with an optional shortcut that popsup offering to delete all related partitions (which still may not be all data on disk). Further, a summary pops up and shows what will be deleted in red text.

In Guided, user either must set each partition to be deleted, or click on the disk (make model number) and set it to delete which then shows all partitions being set to delete as well.