Why these combinations? See this article
Number of decimals in calculation output:
CPM is the cost of 1,000 impressions (literary Cost Per Mille).
A related metric is eCPM, effective CPM, which is a product of CPC (Cost Per Click) and CTR (Click-through Rate) divided by 1,000.
CPC is Cost Per Click, also known as PPC (Pay Per Click).
CPA is Cost Per Acquisition or Cost Per Action. Also known as CPO (Cost Per Order) or CPL (Cost Per Lead), depending on the context of what you are reporting.
Cost could be calculated from both CPM&Impressions, CPC&Clicks or CPA&Acquisitions. In this calculation we get cost from CPM and Impressions.
|Cost||=||CPM x Impressions|
The number of impressions can be calculated when you have both the total oost and the CPM.
|Impressions||=||Cost x 1000|
The number of clicks can be calculated when you have both the total oost and the CPC.