The Type of Service field (also known as Differentiated Services) in IPv4 records the different priorities of IP traffic. Certain services need to be prioritized to enable efficient network utilization. Services such as voice services (for example, IP telephones) need to be relatively real-time, where web browsing might be allowed on a bigger delay. The Type of Service field allows each type of service to identify itself, and therefore routing decisions can take this into account (for example, when load balancing). 

In IPv6, the Traffic Class field performs the same function.