Until last Saturday, advertisers wanting to buy an ad on Snapchat had to negotiate directly with the company’s ad sales team. But starting Oct. 1, Snap, the app’s owner, began allowing some advertisers to bid on ad space using automated systems run by outside ad tech firms. It was a long-awaited move that makes it easier for a wider range of advertisers to buy spots on Snapchat and do more precise targeting.

But Snap isn’t throwing wide the doors anytime soon, as it endeavors to control the kind of ads that appear on its app as well as the kind of software that can integrate with its servers. It’s a tricky balancing act for the company, which is trying to boost ad revenue sharply ahead of an expected IPO filing in coming months.