Cost Per Mille, more often referred to as CPM or Cost Per 1,000 Impressions, is one of the most commonly used metrics in advertising. It measures relative cost of advertising to a given audience. It literally means cost of 1,000 views of an advertisement by a target group.CPM Program in online advertising terminology refers to an arrangement when advertiser's cost and therefore publisher's compensation is directly linked to the number of ad views or impressions.CPM program should also specify whether your advertisement is going to be shown once or several times per user. Unique impressions generally cost more as they cover a wider audience. It is however reasonable to purchase non-unique impressions at a lower or discount rate as well since it takes some users two or more impressions of the same advertisement to take action.One of the most important factors of a CPM based advertising is ad placement location. Ads that are placed in highly visible locations will convert better than ads placed on the bottom of a page for example. It is imperative that CPM-based ads are placed above the fold to provide a fair return on investment. expo-MAX requires that all publishers place their CPM based ads above the fold.