What's the main role of DSCP in QoS?

DSCP’ s Role in QoS:

DSCP is the six most significant bit of the DiffServ (DS) field. The DiffServ architecture defines this DS field to make per-hop behaviour (PHB) the decision about packet classification and traffic conditioning a function such as metering, marking, shaping and policing.

There are three ways we can use DSCP field

1. Classifier: Select a packet based on the contents of some portions of the packet header and apply PHB based on service characteristics defined by the DSCP value
2. Marker: Set the DSCP field based on the traffic profile
3. Metering: check compliance to traffic profile using either a shaper or dropper function.

Note:

DiffServ is a computer networking architecture that specifies a simple and scalable mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks. DiffServ can, for example, be used to provide low-latency to critical network traffic such as voice or streaming media while providing simple best-effort service to non-critical services such as web traffic or file transfers. (Reference: Wiki)