Yes the QoS notification is mainly for kernel drivers, but
applications can control them using the /dev/[...,network_latency,...]
interface as documented in Documentations/power/pm_qos_interface.txt
The device drivers are expected to get feedback (tunable?) from
applications that are dependent on those drivers and set the correct
power saving level. Multimedia applications are expected to make use
of this interface to set/communicate the correct power saving levels
for audio drivers.
Many application can set different latency requirement, but the least
will win. Here the PM-QoS framework in kernel arbitrates between
applications and resolves conflicts by choosing the least latency or
most conservative power saving mode.