Hi!
I need to know what is use of mark routing in ip-firewall-mangle on RouterOS?
what can be done with this option?
Thanks again! 
It’s used to implement policy routing whereby you can have the router use different routing tables based on anything you can match in a mangle rule. For example, you could have SMTP traffic use a default router different from that of other traffic. Refer to the manual for examples.
I mark the connection then mark routing to send all http traffic via a different modem.