Hi Edrisse ... when I draw arrows, I usually draw them from active thing to passive thing. For example, "A calls B", or "A uses B". There's nothing to prevent you from drawing arrows the other way around though (e.g. "data flows from B to A") if you like, just make sure that you annotate the arrow to describe the intent.
To answer your specific question, sure, why not. For example, perhaps your external system is providing data to a container (e.g. an application server). Or maybe that external system is calling an API or sending messages, to which your application server is subscribing. As I said, just make sure you annotate the arrow to describe the intent. Hope that helps!
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