Chapter 1 What’s New for FortiOS 5.2.1 : Other New Features : TOS and DSCP Support for Traffic Mapping
  
TOS and DSCP Support for Traffic Mapping
Both TOS and DSCP are now supported for traffic mapping but only one method can be used at a time, with TOS as the default. The type used and its other attributes can be configured through the CLI.
Syntax
config system global
set traffic-priority {tos | dscp}
set traffic-priority-level {low | medium | high}
end