Final answer:
Multiple data flow lines between symbols on a DFD are typically needed when data elements flow back and forth between the same data store, to represent distinct and interactive flows of data.
Step-by-step explanation:
The student's question asks about when to use multiple data flow lines in a Data Flow Diagram (DFD). In a DFD, more than one data flow line between symbols is typically needed when data elements flow back and forth between the same data store. If data elements are flowing separately or together but in a single direction, a single arrow may suffice. However, if there is an exchange or cyclical flow of data elements, multiple arrows are used to represent the nature of this interaction clearly.
It's important to remember the guideline for using single or multiple data flow lines to accurately depict the system analysis. Each line should represent a unique and distinct flow of information within the system being modeled. For instance, if a foreign country is involved in a transaction, as suggested in the example provided, different arrows will illustrate imports and payments, exports, and the flow of payment for imports, to clearly show the direction and relationship of each data element.