I'm not sure what you're referring to.
Usually 'external data sources' refer to databases. Some maps use specific libraries to write meta data into the map which can be recorded in the replay file. Then someone could upload their replay to have their data tracked--e.g. kill streaks, win/loss ratio, anything.
But it isn't that easy unless you know what you're doing. Other than that, I'm not sure what "external data source" refers to.