@Andre_BR 

It's basically an option that can be used if you are having trouble getting a DHCP address from your ISP.  if you have it unchecked and are getting an address from your ISP you can leave it unchecked.  Checking or unchecking only effects the WAN connection to your ISP.

There are other vendors that use the same option:

Use Unicasting:  This option is normally turned off, and should remain off as long as the WAN-side DHCP server correctly provides an IP address to the router. However, if the router cannot obtain an IP address from the DHCP server, the DHCP server may be one that works better with unicast responses. In this case, turn the unicasting option on, and observe whether the router can obtain an IP address. In this mode, the router accepts unicast responses from the DHCP server instead of broadcast responses.