What is the Difference between "Internal" and "External" Source Systems in Data Warehousing? I am reading about data warehousing and I run into the concept of internal and external source systems.  


  Data are extracted from various internal and external source system files and databases.




I couldn't clearly distinguish between the concept of internal and external.  

Any explanation of the key difference?

Thank you,
You could get data from data providers - for example, geoip data from Maxmind, rating data from Moody's, device data from BlueCava. Those are all external systems. Systems used by the organization are internal systems.