How works "NV2:built-in QoS policy algorithm"?

From http://wiki.mikrotik.com/wiki/Manual:Nv2
Nv2-qos=default
In this mode outgoing frame at first is inspected by built-in QoS policy algorithm that selects queue based on packet type and size. If built-in rules do not match, queue is selected based on frame priority field, as in Nv2-qos=frame-priority mode.

But, how works “built-in QoS policy algorithm”??
I need to priorize some packets, but If I use "Nv2-qos=frame-priority " I don’t know know how it affects the not prioritized traffic by me and I’m afraid of losing efficiency for it.

I need to know what makes “built-in QoS policy algorithm” to priorize, but I can not find documentation about this.
Someone can help me?

Thanks