Hi Dhanya,
RTB, is an open auction marketpalce where all the bids takes place automatically on the ad exchanges via DSPs without any human intervention and the highest bid wins the impressions whereas in RTB (Direct) it requires human intervention as the publisher sales rep needs to negotiate a rate with an advertiser directly and publisher provides a deal ID which needs to be plugged in within the DSP for running the campaigns. Although the process doesn't include require much human intervention but is more manual than RTB.
Hope this helps.
