How to prepare a Delta type Flat File Loading

Hi,

I have one where we have to load data using flat file and this flat file will have 90 Days data.

We will receive this file everyday and after PSA till staging DSO we want to make it full then it should be delta to next DSO for further loading.

So need your help as what should be Key to make this flat file loading delta like using 0Posting Date in Info Package or System Date..

Eagerly waiting for your response.