Twitter is the latest app to reach out to advertisers with an advisory about the just-released data tracking rules on Apple devices, and it is telling brands to brace for disruptions to ad campaigns.

On Tuesday, Twitter apologized for the “short timeline around these product updates,” but that the company was still digesting all the information from Apple about iOS 14.5, the latest iPhone software, and the “app-tracking transparency” rules, known as ATT. Twitter gave guidance to marketers on how to deal with the changes, since it will affect how ads are targeted and measured, and how much data brands can use for both those functions.

Twitter’s ATT impact report revealed what many advertisers have expected since last year, when Apple announced that it would roll out the new data collection rules. Apps like Twitter, and the developers that advertise on Twitter, Facebook, Snapchat, Google and elsewhere, have to get permission from users to track their off-app activity. In recent weeks, Apple iPhone users have been seeing more prompts from the apps asking for that permission to track them for the purposes of personalized advertising.

Twitter said on Tuesday that it will start showing users a data permission notice within the next two weeks. The fear is that most consumers will opt out of tracking and it is likely to affect how marketers conduct targeted advertising on Apple devices.

“We expect these updates will impact products in different ways, and it will be important to understand and prepare for any changes to products you currently use,” Twitter said Tuesday.

Apple’s data collection changes should have minimal impact on broad-based brand campaigns, Twitter said. That’s because advertising for brand awareness goals, like informing masses of people of a product launch, doesn’t require hyper-specific targeting, the kind powered by data that Apple is restricting.

However, the more direct-response marketing will be affected, the kind of ads that rely on targeting to hit the consumer most likely to click on a link, download an app, or purchase a product. Apple’s changes are likely to affect the ability of marketers to bring email lists and other targeting information into apps like Twitter to reach “custom audiences,” Twitter said.

The warning was similar to ones issued by Facebook and Snapchat in recent weeks. Meanwhile, targeting ads to audiences based on data that Twitter owns is likely to continue uninterrupted, but using data from outside third-parties could be hampered.

“As people upgrade to iOS 14.5+ and do not opt in to tracking, we will see a reduced ability to target users based on age and gender across the [Twitter Audience Platform] ecosystem,” Twitter said. “TPA requires [Apple’s Identity for Advertisers] in order to identify age and gender. With reduced IDFA signal, using age and gender targeting with TAP will heavily restrict accessible reach across TAP.”

Twitter had recommendations for brands to increase the reach of their ad campaigns by using “lookalikes,” which are consumers who resemble, based on their web activity, the audiences an advertiser already knows.

Twitter told brands to use “lookalikes” of custom audiences. It also advised brands to set targeting features to “broad.”

Twitter also warned that it will be difficult to report on “conversions,” because less tracking data means advertisers can’t see exactly when an ad led a consumer to take an action, like purchasing a product or visiting a website outside of Twitter.

“We expect to see a significant decrease in the volume of conversions reported from iOS users,” Twitter said. “As we have more to share, we will update you with our plans to begin reporting on conversions from people using iOS devices who have opted in to tracking.”

Twitter said it will be working with Apple’s new measurement protocols through the SKAdNetwork, which is Apple’s platform for advertisers to analyze campaigns while still adhering to the privacy guidelines.

“Up until very recently, there were still unknowns our team was navigating with Apple’s policy and enforcement, which in turn affected the impact and product changes we could interpret,” Twitter said. “We sincerely apologize for the short timelines around these product updates.”