CPM means cost per mille, that is a cost of a thousand globally unique impressions. It's counted like:

CPM = all spent money / all unique impressions * 1000

So max CPM is the maximum price you're ready to pay for the traffic. It doesn't mean that your actual CPM will be equal to your max CPM, although it's possible. Max CPM just means that you're not ready to pay higher CPM for the traffic.

Our default max CPM is 15 USD.

You might want to set different max CPM. It can be done at https://a-ads.com/campaigns/YOUR_CAMPAIGN_ID/payment_model/edit where YOUR_CAMPAIGN_ID should be replaced by your actual campaign id.
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