Defining the metrics
In online advertising there is a lot of concepts and terms that would be difficult to understand for those not familiar with it.
CPM is a relatively easy concept, only influenced by the number of impressions and the media cost. But when things like clicks, acquisitions, leads, CPA, CPL, CPC, CPO and eCPM starts to appear in your equations, things might get ugly.
In a lot of cases there will be some overlap between the different metrics. In general you could say that CPA, CPL and CPO is the same metric. CPM, eCPM and RPM is somewhat also the same metric, as well as CPC, eCPC and PPC.
To better understand the theoretical difference between terms like CPM and eCPM or between CPC and eCPC, I recommend you take a look at the buying methods in online advertising.