Search Impression Share is the percentage of impressions won by an ad campaign or ad group or keywords, out of the total impressions the campaign is eligible for. Ad targeting, keywords, and ad scheduling all play a role in the SIS won.

Search Lost Impression Share (Rank) is the percentage of search impression share lost out of the total eligible impressions, due to Ad Rank. Ad Rank depends on factors such as relevant keywords and targeted ads.

Spend to Maximize Impression Share describes the investment level or budget an ad campaign needs to maximize Search Impression Share with its current targeting settings. It is based on the idea that an ad campaign’s spend or budget controls the frequency with which an ad is displayed.

Search Lost Impression Share (Budget) is the percentage of Search Impression Share lost out of the total eligible impressions, due to campaign budget. It is generally measured at the campaign level on digital advertising platforms.