Currently dnf and yum have their own yumdb's, under /var/lib/dnf/yumdb and /var/lib/yum/yumdb respectively.  Among other things, these databases record whether a package was explicitly installed by the user or pulled in automatically as a dependency of another package. This information is used by the "dnf autoremove" and "yum autoremove" routines to determine which additional packages can be safely removed. As Packagekit currently writes to yum's yumdb, "dnf autoremove" will not work correctly when run on packages that were installed using Gnome software, and vice versa.  

Thus we should consider merging the yumdb's as part of the F22 transition to a unified dnf/hawkey package management backend. From a brief inspection, the two yumdb's on my system appear to have the same structure. All that would need to be done is decide how to proceed in the rare event when the same package appears in both yumdb's. On my machine, make and pkgconfig show up in both yumdb's with the install reason marked as "group" by dnf and "dep" by yum. Since dnf knows more than yum due to its tracking of groups (http://dnf.baseurl.org/2014/04/23/how-groups-are-handled-in-modern-day-dnf/), I'd be inclined to let dnf's version take precedence.

Best,

Casey