'Mark' is where your service policy sets the DSCP value in line with the policy. i.e. incoming voice packets (identified by an ACL) have the DSCP value set to 46.
'Markdown' is where you have a policer within your service policy and packets that do not conform to the policer will have the DSCP value changed. Other options of the policer are to drop the packets.
The Markdown behaviour is useful when you have a high-bandwidth Campus LAN infrastructure where dropping or delaying packets is unlikely. If the packets must exit the Campus and use low-bandwidth links then the DSCP value can be used to determine traffic that is 'inside' and 'outside' of the policy - packets that have been marked down can be given less priority or dropped than packets with 'good' DSCP values.
Have a search on Scavenger Class for a good explanation by Cisco.
Andy
