Identifies the data source. As function of the type entered previously, this field can be :
- a multi-list code (MLT source type).
- the code of a request, or a SQL request (REQ source type).
- The code for a statistic (STA data source).
- The code for a process (TRT data source).
- empty (the case for the URL and AGD source type, for which only one list of parameters is required).