The difference between CPA and CPS offers is that CPS offers can have several conversions per user click ID.
In typical configuration (CPA offers), through the same user click ID, only one conversion is possible (i.e. if a conversion has been made, even if the postback will be fired, another conversion will not be recorded).
In order to count the number of consecutive conversions on an CPS offer (where the conversion of a user is counted as: registration, then, ordering of a product and a payment, for example), you need to choose the CPS offer type, and add the parameter txid to the postback. In this case, the conversion will be counted at a unique combination of tid + txid.
In the “Conversion report” section, you can see the value of conversions made through tid+txid.  

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