Hello.
After filling all the information related to data sources, you will fill the indicators, specifying the information related to the availability of the indicator which is produced by a specific data source. Doing so, you will have to select the data source which producte the indicator, linking the indicator to the data source. Since it is likely that you will fill the indicator after having located it in a logframe, you will automatically link the data source to the logframe that contains the indicator.
Note that you could fill the data source for an indicator only if you have stated that the indicator is available in the country.