Also known more formally as SERVICE TYPE. The TOS is an 8-bit field in a datagram, or IP packet header. It was originally divided into five subfields (precedence, D, T, R, and unused). The IETF redefined the field in the 1990's to accomodate a set of differentiated services (DS), containing the 6-bit codepoint (DSCP) and 2 bits that are unused. Multiple codepoints map to each service. Backward compatibility is maintained by distinguishing between the first and last 3 bits of the codepoint.

Basically, the TOS field helps the routing algorithm choose among various paths to a destination based on policies.