Type Of Service (ToS) field
The ToS field (1 byte) is used IP protocol suite to describe the desired quality of service for an IP datagram by upper layer protocols like TCP. This field can be used to specify the nature and priority of a IP datagram (like Network Control, Immediate, Critical, etc) and the criteria for selecting a path for forwarding a datagram by a gateway. RFC 1349 discusses the Type Of Service (TOS) field.
Unless you exactly know what you do, don't create rules checking Tos field.