Two kinds of priority information are token into consideration when a packet is processed. One is COS, the IEEE 802.3ac tag containing IEEE 802.1p priority information, and the other is DSCP, the IPv4 Type of Service/DiffServ field. (1)COS only: the port priority will only follow the COS priority that you have assigned. (2)DSCP only: the port priority will only follow the DSCP priority that you have assigned. (3)COS first: the port priority will follow the COS priority first, and then other priority rule. (4)DSCP first: the port priority will follow the DSCP priority first, and the other priority rule.