The two most commonly used QoS labels in the Layer 3 IP header are the IP precedence field and the DSCP field. The QoS label in the Layer 2 frame header is called Class of Service (CoS). Catalyst switch QoS tools can provide the preferential treatment based on either Layer 3 QoS labels or Layer 2 QoS labels.
MLS is Multilayer Switching.
mls qos trust device = configure the port to trust a Cisco phone tagged packet. (COS or DSCP)
ToS/differentiated services code point (DSCP)
This apply the trust boundary up to the phone.
This is used for QoS config later, when you have to trust COS or DSCP flag in you LAN, and apply priority queuing or policy or shapping or whatever you dream to do
Type of Service (ToS), cisco info
The use of multiple queues on transmit interfaces is the only way to eliminate the potential for dropped traffic caused by buffers that operate at 100% capacity. The separation of voice and video (both sensitive to delays and drops) into their own queues can prevent flows from being dropped at the ingress interface, even if data flows fill up the data transmit buffer.