We have used the program’s control flow as a way to derive different tests. Another way of approaching structural testing is to look at the data flow: examining how the data flows to different parts of the program.
For example, imagine that a variable is defined, then modified one, two, or three times in other parts of the program, and then used again later. You may want to ensure that you exercise all the possible ways this variable is touched. Trying to summarize data-flow coverage in one sentence is unfair, and a lot of energy has been spent coming up with criteria, but this should give you some intuition about it.
I suggest you read more about it. Pezzè and Young (2008) give a nice explanation.
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