LR 6 (you buy it) is the upgrade to LR 5. New features, just like upgrading from LR 4 to LR 5.
LR CC (subscription) is LR 6 where you pay a monthly subscription. LR CC gets you additional newer features that LR 6 users need to wait for LR 7 to get.
LR 6/cc will migrate your LR 5 catalog to the new LR 6 format.
LR 6 cannot directly use your LR 5 catalog.
LR 5 cannot use your LR 6/cc catalog.
Thank you for your fast answer. So then i can delete LR5 and all the images will remain on my harddisk right?
Yes, as long as you are careful about what you delete.
After you uninstall LR5, you can delete the LR5 version of the catalog (*.LrCat) and its preview folder (*.LrData).
Just make sure you don't delete the LR6 catalog and previews (different names, same extensions).
Your images are not "in" Lightroom; they are on your hard drive and just referenced by Lightroom using full paths. As long as you do not delete the images, you are fine.