QoS - marking
Within the framework of QoS marking is the process by which we mark or label, or earmark so to speak, a packet of data based on the classification of the particular traffic. Marking can occur at either Layer 2 or Layer 3 or both.
At Layer 2, we can mark the Class of Service (CoS) value that is contained within the 802.1Q VLAN tag. Note that this is done on ports configured with a voice VLAN and on trunks since it is on frames sent from such ports that VLAN tags exist.
For more information, take a look at QoS CoS vs DSCP.
Once marked, traffic is not yet acted upon by any QoS policies. In order to be acted upon, QoS policies must be applied.