IPv6 has a traffic class field. According to the specification it is an IPv6 length, out of which first 6 bits are called code point or differential services code point (DSCP) and the last 2 bits are used for explicit congestion notification(ECN). These 6 bit combinations (DSCP) are used to classify IP packets so that one class of IP packet can receive precedence over the other in a network.
IPv4 has a field, called type of service (ToS). According to some references it is also used to carry DSCP and ECN bits (as for example explained in a video). However according to my professor IPv4 packets have no priority, which implies that this field is used for something else.
What is correct?