What is the difference between D-Bus policy files and SELinux?

D-Bus policy files, typically XML under /etc/dbus-1/system.d/ or /usr/share/dbus-1/system.d/, are D-Bus's own, application-level access control mechanism: they define rules like which UID or group can call which method on which interface and destination, enforced entirely by the bus daemon itself.

SELinux, by contrast, is a kernel-level mandatory access control system that applies to far more than D-Bus, labeling processes and resources with security contexts and enforcing rules independent of any single application's own logic. On systems where SELinux is enabled and D-Bus-aware, it can add an additional layer of enforcement on top of D-Bus policy files, checking whether the calling process's security context is permitted to communicate with the destination's context at all.

In practice the two are complementary rather than competing: D-Bus policy answers "is this specific method call allowed by this service's rules," while SELinux answers a broader "is this process even allowed to talk to that process," and a request can be blocked by either layer independently.