Flags campaigns that are losing a meaningful share of impressions purely because they've hit their budget - in other words, there's demand you're leaving on the table. It's channel-aware, reading the right signal for each campaign type: Search and Shopping via the share of impressions lost to budget over the latest complete week, and Performance Max, Display, Video and Demand Gen via Google's own budget-constrained status.
Defaults & settings
Lost impression share threshold - flag Search/Shopping campaigns losing more than this percentage of impression share to budget. Default 20%, adjustable 5-50%.
Checks daily; 72-hour cooldown.
Good to know. A budget-limited flag is a decision point, not automatically a "spend more" - check the campaign's CPA or ROAS against target first. If it's performing under target, more budget genuinely helps; if not, the constraint may be doing you a favour. The mirror image - a campaign losing share for other reasons - is the job of Impression share drop.