This sets an alternate TOS/DSCP marking used for packets that should be treated with a higher drop precedence than the baseline TOS value configured elsewhere in this pseudowire's configuration, for implementations that support two-level drop precedence marking; if not supported, it simply mirrors the baseline TOS value. Administrators use this on devices that support differentiated drop handling to mark certain pseudowire packets (for example, less critical or excess traffic) so they're preferentially dropped under congestion before higher-precedence packets. Setting this object to a DSCP value associated with lower priority than the baseline would mean packets carrying this marking are more likely to be dropped under congestion than packets marked for higher drop precedence.